iio:trigger: modify return value for iio_trigger_get

Instead of a void function, return the trigger pointer.

Whilst not in of itself a fix, this makes the following set of
7 fixes cleaner than they would otherwise be.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org