iio: gyro: bmg160: remove redundant field

Replace the 'timestamp' field in struct bmg160_data with the identically
named field in iio_poll_func and with calls to iio_get_time_ns().

The reported timestamps may be slightly different, but the advantage is
that we no longer assume that the buffer of bmg160 is triggered by its
own trigger.

Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>