mixer_group: improved get_multirotor_count () code readbility.
mixer_group: improved get_multirotor_count () code readbility.

Signed-off-by: Claudio Micheli <c3923186c4c9da9c766af46f22a325cd4677020a@auterion.com>

acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,dagar/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,dagar/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,dagar/Firmware,PX4/Firmware,krbeverx/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,dagar/Firmware,dagar/Firmware,dagar/Firmware,PX4/Firmware