regulator: max77686-regulator: 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/max77686-regulator.o
   text	   data	    bss	    dec	    hex	filename
   9904	   1320	      0	  11224	   2bd8 regulator/max77686-regulator.o

File size after: drivers/regulator/max77686-regulator.o
   text	   data	    bss	    dec	    hex	filename
  10928	    280	      0	  11208	   2bc8 regulator/max77686-regulator.o

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