regulator: Assign return value of mc13xxx_reg_rmw to ret

Otherwise, we will not return error if write to MC13892_SWITCHERS5 failed.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Yong Shen <yong.shen@linaro.org>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>