altos: Configure packet size from send/recv parameters.

Instead of setting the packet size at configuration time, use the
provided packet size to the send/recv functions to configure the
radio. This eliminates many configuration calls, leaving us with 'RDF'
mode and 'packet' mode, the latter working for telemetry and the
bi-directional link.

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