regulator: core: add const to regulator_ops and fix build error in mc13892

Commit 272e2315

 ("regulator: core: add const qualifier to ops in
struct regulator_desc") introduced const qualifier to ops in regulator_desc.

This patch adds 'const' to regulator_ops vars in newly added core APIs
for v3.17-rc1:
 - regulator_get_hardware_vsel_register()
 - regulator_list_hardware_vsel()

This patch also fix a build error in mc13892-regulator.c due to const
regulator_desc.ops. Modification of regulator_desc.ops' member fields is not
allowed.

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