AP_HAL: fixed a build warning
AP_Airspeed: fixed get_temperature on PX4
HAL_PX4: don't enable IFLOW on ports without flow control

this should have been harmless, but breaks the GPS on Pixhawk. This
disables it until we work out why.
HAL_PX4: better fix for IFLOW on all ports

the tcsetattr() call fails to apply the baudrate if the port has no
RTS pin and we ask for IFLOW. So just make a separate call.
Copter: add RC_FEEL_RP parameter

Values from 0 ~ 100 control amount of filtering on roll and pitch input.
100 = no filter so crisp feel, 0 = a lot of filtering so very sluggish
response