iio: gyro: bmg160: fix buffer read values

When reading gyroscope axes using iio buffers, the values
returned are always 0. In the interrupt handler, the return
value of the read operation is returned to the user instead
of the value read. Return the value read to the user.

This is also fixed in commit 82d8e5da1a33 ("iio:
accel: bmg160: optimize transfers in trigger handler").

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>