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 the declaration of device_type structure.

File size before: drivers/hid/intel-ish-hid/ishtp/bus.o
   text	   data	    bss	    dec	    hex	filename
   4260	    336	     16	   4612	   1204 hid/intel-ish-hid/ishtp/bus.o

File size after: drivers/hid/intel-ish-hid/ishtp/bus.o
   text	   data	    bss	    dec	    hex	filename
   4324	    272	     16	   4612	   1204 hid/intel-ish-hid/ishtp/bus.o
Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>