regulator: ltc3589: 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/ltc3589.o
   text	   data	    bss	    dec	    hex	filename
   2564	   3312	    288	   6164	   1814	drivers/regulator/ltc3589.o

File size after: drivers/regulator/ltc3589.o
   text	   data	    bss	    dec	    hex	filename
   3604	   2544	      8	   6156	   180c	drivers/regulator/ltc3589.o

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