mavlink: create stream even if rate is 0
mavlink: create stream even if rate is 0

This allows to send a message exactly once if requested.

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