regulator: Kill s5m8767_get_reg_id function

Calling s5m8767_get_reg_id() is exactly the same as calling rdev_get_id().
It is pointless to add s5m8767_get_reg_id() function.
Use rdev_get_id() directly and remove s5m8767_get_reg_id() function.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>