altos/kalman: Kalman terms can be > 1, use 32-bit fixed point

altos/kalman: Kalman terms can be > 1, use 32-bit fixed point

Because speed and acceleration are scaled by 16, it's fairly common
for the kalman terms to end up larger than 1. Instead of trying to
fuss with 16-bit values and shifts, just use 32-bit values.

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