regulator: as3722: Constify regulator ops

A const pointer to regulator ops is stored in regulator descriptors. The
operations never need to be modified, so define them as const as a hint
to the compiler that they can go into .rodata.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>