altos: Rework packet receive for cc1120

altos: Rework packet receive for cc1120

Instead of blocking on PQT, just set up the receiver to start going
and when the first bit interrupt comes in, grab the SPI bus if
possible and configure it for reception. This improves sensitivity in
the radio by a significant amount while making the code conceptually a
bit nicer.

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