altos: Clear any broken cc1120 TX fifo bits before transmitting

altos: Clear any broken cc1120 TX fifo bits before transmitting

This just goes and clears the transmitter before using it, just in
case it got wedged somehow. It also clears the bits while waiting for
the radio to go idle, otherwise it'd never make it.

Signed-off-by: Keith Packard <keithp@keithp.com>