SERVOx_FUNCTION: run out of options using AP_Int8

[IamPete1]

We have now filled the number of servo output functions available using a Int8 parameter.

ardupilot/libraries/SRV_Channel/SRV_Channel.h

Line 153 in d844592

k_yaw_out = 127,

ardupilot/libraries/SRV_Channel/SRV_Channel.h

Line 239 in d844592

AP_Int8 function;

Int8 has a max value of 127, if we were to go to Int16 that would give us 32767.

I attempted a fix but failed on the parameter conversion.

[rmackay9]

yes, parameter conversions can be tough. RC_Channel has some param conversion code that might be helpful but it's a guess. we should probably add a developer wiki page at some point with advice.