fix rc.interface: do not append .mix when setting MIXER_AUX
fix rc.interface: do not append .mix when setting MIXER_AUX

Because further down the mixer file is set as:
	set MIXER_AUX_FILE /etc/mixers/${MIXER_AUX}.aux.mix

PX4/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,dagar/Firmware,dagar/Firmware,Aerotenna/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,PX4/Firmware,Aerotenna/Firmware,PX4/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,Aerotenna/Firmware,acfloria/Firmware,dagar/Firmware,mcgill-robotics/Firmware,PX4/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,acfloria/Firmware,jlecoeur/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,dagar/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,krbeverx/Firmware,jlecoeur/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,PX4/Firmware,acfloria/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware