iio: bmc150: refactor slope duration and threshold update

Move the slope duration and threshold update in a separate function to
reduce code duplicate between chip init and motion interrupt setup.

Also move the slope update code from the interrupt setup function to
the trigger set state function so that we can later refactor the
interrupt code.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>