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

File size before:
   text	   data	    bss	    dec	    hex	filename
  17184	   1344	     80	  18608	   48b0	drivers/input/input.o

File size after:
   text	   data	    bss	    dec	    hex	filename
  17248	   1280	     80	  18608	   48b0	drivers/input/input.o

File size before:
   text	   data	    bss	    dec	    hex	filename
   2355	    384	      8	   2747	    abb	drivers/input/rmi4/rmi_bus.o

File size after:
   text	   data	    bss	    dec	    hex	filename
   2483	    264	      8	   2755	    ac3	drivers/input/rmi4/rmi_bus.o
Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>