altos/cc1200: Wait for packet to be placed in FIFO during receive

This changes the receive code to use MCU_STATUS, waiting for
MARC_STATUS1 to indicate that the packet is in the fifo before reading
it out.

It also fixes the receive timeout code to keep receiving if the
preamble or sync have been seen when the timeout fires. This makes
TeleLCO able to use short timeouts during scanning while still
successfully receiving packets.

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