fix(usb): Fix -Wincompatible-pointer-types compiler error

[mlouielu]

Fix #2794, where get_keyboard_report(len) didn't convert len from int32_t * to size_t *.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.

[joelspadin]

This doesn't actually fix the issue that the compiler is warning you about. It just says, "I know what I'm doing", so the compiler stops warning about it.

len is still a pointer to int32_t. Casting it to a pointer to size_t means that get_keyboard_report() is going to write a size_t to the memory that is actually an int32_t. On a platform such as native_posix_64 where size_t is uint64_t, it will write a 64-bit value to the given address, but the variable is only 32 bits, so it clobbers some memory after it.

Some safe alternatives:

• Change get_keyboard_report() to also use an int32_t for its output parameter (requires also updating any other places that call the function to use int32_t as well).
• Create a new size_t variable here, pass its address to get_keyboard_report(), then assign its value to len.