iio: imu: Use iio_trigger_get for indio_dev->trig assignment

iio: imu: Use iio_trigger_get for indio_dev->trig assignment

This patch uses iio_trigger_get to increment the reference
count of trigger device, to avoid incorrect assignment.
Can result in a null pointer dereference during removal if the
trigger has been changed before removal.

This patch refers to a similar situation encountered through the
following discussion:
http://www.spinics.net/lists/linux-iio/msg13669.html

Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>