uavcan_virtual_can_driver: fix invalid use of px4_sem_getvalue
uavcan_virtual_can_driver: fix invalid use of px4_sem_getvalue

sem_getvalue returns 0 on success, -1 on error, and never a value > 0.

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