iio: accel: bmc150: decouple buffer and triggers

If the interrupt pins are not available, we should still be able to use
the buffer with an external trigger.  However, we won't be able to use
the hardware fifo since we have no means of signalling when the
watermark is reached.

I also added a comment to indicate that the timestamps in
bmc150_accel_data are only used for hardware fifo, since initially I was
confused about duplication with pf->timestamp.

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