regulator: core: Add back the const qualifier for ops of struct regulator_desc

Fix below build warning:
CC [M]  drivers/regulator/hi6421-regulator.o
drivers/regulator/hi6421-regulator.c:356:2: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]

This is a revert of commit 716845eb ("regulator: core: Fix build error due
to const qualifier for ops"). The build error was fixed by commit 39f5460d


("regulator: core: add const to regulator_ops and fix build error in mc13892").

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>