altos: Reduce deviation at 2400/9600 baud

There's no reason to use 20.5kHz deviation at 2400 and 9600 baud, and
if we get a better receiver, we'll want to use narrower deviation to
have less bandwidth sucking noise into the radio. The new values are
(nominally) 5.125kHz deviation for 9600 baud and 1.5kHz deviation for
2400 baud.

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