Currently the bma421 driver simple re-initializes the sensor when asked
to set the step counter to zero. Switch over to the proper function
for this.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

There nothing in the docs to give the delay time required after a
reset. Currently we use 200ms because that appears on some older
code for BMA423 but is removed in more recent drivers. 50ms is still
a long time (for hardware) and has held up in testing.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

Currently there's no fancy algorithms to estimate stride length. Just
pure simple step counting directly from the hardware's "intelligence
engine".

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>