RX.cpp: disable normal RC input when using RC_TINY

Instead of invoking failsafe on BLE link loss it will reset throttle to the midpoint (e.g. 1500) making the copter shoot up in the air.

See https://github.com/koenkooi/multiwii-firmware/issues/17

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>