RC_Channel: fixed dead_zone for range channels

The dead_zone was being used inconsistently, used in PWM space in one
place, and in control output space in another.

The fix required us to move the index number of the RC channel eeprom
value for RCn_DZ, as users will have a throttle deadzone of 3 set in
their eeprom due to a bug that Randy just fixed that caused the value
to always be saved to eeprom. In plane we then need to fix the
deadzone for the throttle to be 30

this fixes issue #303

Thanks to Soren Kuula for spotting this!