regulator: core: add const qualifier to ops in struct regulator_desc

struct regulator_ops *ops is a member in struct regulator_desc, which gets
its value from individual regulator driver upon regulator_register() and
is used by regulator core APIs. It's not allowed for regulator core to
modify any of these callbacks in *ops. Add 'const' qualifier to enforce that.

Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>