altos: Don't abort radio transmissions with ao_radio_abort

We only want to abort pending radio reception to release the radio for
other use, or to change the radio channel. Let radio transmission
proceed. This fixes a problem with using packet mode to configure the
radio channel; if the packet transmission is aborted, the TM ends up
wedged.

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