fmu: fix init & use of _control_subs
fmu: fix init & use of _control_subs

0 is a valid file descriptor, init (all) to -1 instead

dagar/Firmware,Aerotenna/Firmware,PX4/Firmware,PX4/Firmware,Aerotenna/Firmware,acfloria/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,dagar/Firmware,Aerotenna/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,krbeverx/Firmware,dagar/Firmware,krbeverx/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,Aerotenna/Firmware,dagar/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware