Declare regulator_ops structure as const as it is only stored in the ops
field of a regulator_desc structure. This field is of type const, so
regulator_ops structures having this property can be made const too.

File size before: drivers/regulator/aat2870-regulator.o
   text	   data	    bss	    dec	    hex	filename
    938	   1424	      0	   2362	    93a regulator/aat2870-regulator.o

File size after: drivers/regulator/aat2870-regulator.o
   text	   data	    bss	    dec	    hex	filename
   1194	   1168	      0	   2362	    93a regulator/aat2870-regulator.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>