regulator: s2mps11: Reduce number of rdev_get_id() calls

Store the regulator ID instead of calling rdev_get_id() every time.
This makes code slightly easier to read as shorter 'rdev_id' variable is
used instead of full call.  This can also speed things up by reducing
number of calls, although effect was not measured.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>