samples: decrease log level in dfu mode

[ktaborowski]

The debug messages are disabled by default.
Log message might be misleading.

CI parameters

(branch, hash, pull/XXX/head)
NRF_revision=main

(true, false)
Create_NRF_PR=false

Description

JIRA ticket:

Detail description of the change

Self review

• There is no commented code.
• There is no TODO/FIXME comments without associated issue ticket.
• Commits are properly organized
• Verification
  • Unit tests were updated to include the change.
  • Change has been tested.

[github-actions]

Sample		diff	used	total
nrf54l15pdk_nrf54l15_cpuapp:sample.sidewalk.dut	RAM	0 B	121.36 KB	256 KB
	ROM	-68 B	414.82 KB	730 KB
nrf54l15pdk_nrf54l15_cpuapp:sample.sidewalk.hello	RAM	0 B	87.74 KB	256 KB
	ROM	-68 B	350.87 KB	730 KB
nrf54l15pdk_nrf54l15_cpuapp:sample.sidewalk.hello.ble_only	RAM	0 B	87.74 KB	256 KB
	ROM	-68 B	350.62 KB	730 KB
nrf54l15pdk_nrf54l15_cpuapp:sample.sidewalk.hello.no_dfu	RAM	0 B	74.47 KB	256 KB
	ROM	-68 B	332.11 KB	1.48 MB
thingy53_nrf5340_cpuapp:sample.sidewalk.demo.ble_only	RAM	0 B	102.42 KB	440 KB
	ROM	-68 B	390.79 KB	975.5 KB
nrf52840dk_nrf52840:sample.sidewalk.demo	RAM	0 B	119.83 KB	256 KB
	ROM	-68 B	495.21 KB	983.5 KB
nrf52840dk_nrf52840:sample.sidewalk.demo.ble_only	RAM	0 B	104.21 KB	256 KB
	ROM	-68 B	412.76 KB	983.5 KB
nrf52840dk_nrf52840:sample.sidewalk.dut	RAM	0 B	143.21 KB	256 KB
	ROM	-68 B	554.12 KB	983.5 KB
nrf52840dk_nrf52840:sample.sidewalk.hello	RAM	0 B	109.58 KB	256 KB
	ROM	-68 B	494.91 KB	983.5 KB
nrf52840dk_nrf52840:sample.sidewalk.hello.ble_only	RAM	0 B	94.08 KB	256 KB
	ROM	-52 B	412.24 KB	983.5 KB
nrf52840dk_nrf52840:sample.sidewalk.hello.no_dfu	RAM	0 B	96.29 KB	256 KB
	ROM	-68 B	477.11 KB	983.5 KB
nrf5340dk_nrf5340_cpuapp:sample.sidewalk.demo	RAM	0 B	108.09 KB	440 KB
	ROM	-68 B	433.04 KB	975.5 KB
nrf5340dk_nrf5340_cpuapp:sample.sidewalk.demo.ble_only	RAM	0 B	92.89 KB	440 KB
	ROM	-68 B	354.39 KB	975.5 KB
nrf5340dk_nrf5340_cpuapp:sample.sidewalk.dut	RAM	0 B	131.55 KB	440 KB
	ROM	-68 B	491.99 KB	975.5 KB
nrf5340dk_nrf5340_cpuapp:sample.sidewalk.hello	RAM	0 B	98.18 KB	440 KB
	ROM	-68 B	432.82 KB	975.5 KB
nrf5340dk_nrf5340_cpuapp:sample.sidewalk.hello.ble_only	RAM	0 B	82.97 KB	440 KB
	ROM	-68 B	354.13 KB	975.5 KB
nrf5340dk_nrf5340_cpuapp:sample.sidewalk.hello.no_dfu	RAM	0 B	84.95 KB	440 KB
	ROM	-68 B	414.86 KB	975.5 KB
nrf54l15pdk_nrf54l15_cpuapp:sample.sidewalk.demo	RAM	0 B	97.69 KB	256 KB
	ROM	-68 B	351.39 KB	730 KB
nrf54l15pdk_nrf54l15_cpuapp:sample.sidewalk.demo.ble_only	RAM	0 B	97.69 KB	256 KB
	ROM	-68 B	351.14 KB	730 KB