regulator: ad5398: constify regulator_ops structure

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/ad5398.o
   text	   data	    bss	    dec	    hex	filename
   1707	    672	      0	   2379	    94b	drivers/regulator/ad5398.o

File size after: drivers/regulator/ad5398.o
   text	   data	    bss	    dec	    hex	filename
   1963	    416	      0	   2379	    94b	drivers/regulator/ad5398.o

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