regulator: lp8755: 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/lp8755.o
   text	   data	    bss	    dec	    hex	filename
   3519	   2800	      8	   6327	   18b7	drivers/regulator/lp8755.o

File size after: drivers/regulator/lp8755.o
   text	   data	    bss	    dec	    hex	filename
   3779	   2544	      8	   6331	   18bb	drivers/regulator/lp8755.o

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