regulator: act8945a-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/act8945a-regulator.o
   text	   data	    bss	    dec	    hex	filename
   3680	    464	      0	   4144	   1030 regulator/act8945a-regulator.o

File size after: drivers/regulator/act8945a-regulator.o
   text	   data	    bss	    dec	    hex	filename
   3936	    192	      0	   4128	   1020 regulator/act8945a-regulator.o

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