AP_RCProtocol: factor out a poll_protocol method

[peterbarker]

I've tested this with an R-XSR connected to a MatekL431 connected to a CubeRed. Still get RC input.

Should make the RC-input-via-mavlink-packet from @olliw42 simpler.

Board,AP_Periph,blimp,bootloader,copter,heli,iofirmware,plane,rover,sub
CubeOrange-periph-heavy,*,,,,,,,,
Durandal,,8,*,8,8,,8,8,8
Hitec-Airspeed,*,,,,,,,,
KakuteH7-bdshot,,*,*,*,*,,*,*,*
MatekF405,,*,*,*,*,,*,*,*
Pixhawk1-1M-bdshot,,24,,24,32,,24,32,24
f103-QiotekPeriph,*,,,,,,,,
f303-Universal,*,,,,,,,,
iomcu,,,,,,*,,,
revo-mini,,*,*,*,*,,*,*,*
skyviper-v2450,,,,*,,,,,