altos: Reset 3d-accel and motor pressure sums each ground interval

altos: Reset 3d-accel and motor pressure sums each ground interval

We're repeatedly averaging 512 ground samples to get up-to-date values
for all of the sensors, but we need to clear the sums after each
sample interval. The 3d accel values were only cleared if there was a
full IMU, not for devices with only an accelerometer like
EasyMotor. The motor pressure sum was never cleared.

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