regulator: core: Fix build error due to const qualifier for ops

Drop const qualifier for ops of struct regulator_desc.
Allow regulator drivers to update ops before registering regulator.

Fix below build error:
  CC [M]  drivers/regulator/mc13892-regulator.o
drivers/regulator/mc13892-regulator.c: In function 'mc13892_regulator_probe':
drivers/regulator/mc13892-regulator.c:586:3: error: assignment of member 'set_mode' in read-only object
drivers/regulator/mc13892-regulator.c:588:3: error: assignment of member 'get_mode' in read-only object
make[2]: *** [drivers/regulator/mc13892-regulator.o] Error 1
make[1]: *** [drivers/regulator] Error 2
make: *** [drivers] Error 2

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>