sf0x: avoid reopening the UART on startup
sf0x: avoid reopening the UART on startup

And:
- return correct return code in SF0X::init()
- remove unneeded SF0X::probe()

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