fix failsafe behavior after RC loss and mode switch

[haumarco]

Solved Problem

Scenario:
- drone in altitude mode and switches to RTL failsafe after RC loss
- while in RTL, the user selects "position mode", the vehicle command gets rejected because of missing manual mode
- the vehicle switches to altitude mode, despite not having any manual control input!

Solution

I removed the _had_mode_change = true; which always indicated that the "intended mode" has change even when it got rejected.

Now the scenario would look like:
- drone in altitude mode and switches to RTL failsafe after RC loss
- while in RTL, the user selects "position mode", the vehicle command gets rejected because of missing manual mode
- the vehicle stays in RTL
- RC connects again, vehicle stays in RTL until mode gets switched again or switches to position mode if user gives stick input (depending on COM_RC_STICK_OV)

[haumarco]

@sfuhrer @MaEtUgR please also try to break the logic with different scenarios (mode changes, failsafes etc.) that you can think of...