iio: industrialio-trigger: constify device_type structures

Declare device_type structure as const as it is only stored in the
type field of a device structure. This field is of type const, so add
const to declaration of device_type structure.

File size before:
   text	   data	    bss	    dec	    hex	filename
   5389	    208	     48	   5645	   160d	iio/industrialio-trigger.o

File size after:
   text	   data	    bss	    dec	    hex	filename
   5453	    176	     48	   5677	   162d	iio/industrialio-trigger.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>