regulator: lp872x: Kill _rdev_to_offset() function

There is only one user calling _rdev_to_offset() function.
Remove _rdev_to_offset() makes the code simpler.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>