drivers: udc: Do not allow lock/unlock to fail

USB stack does not check api->lock() and api->unlock() return value and
all UDC drivers block without timeout in its lock() and unlock() api
implementations. There is no realistic way to handle lock() and unlock()
errors without making USB device stack API unnecessarily complex.

Remove the return type from lock() and unlock() to make it clear that
the functions must not fail.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>