regulator: Use mc13xxx_reg_write instead of mc13xxx_reg_rmw in mc13892_sw_regulator_set_voltage

Currently, we call mc13xxx_reg_read and mc13xxx_reg_rmw for the same register.
This can be converted to simply a mc13xxx_reg_read and a mc13xxx_reg_write,
thus save a redundant register read.

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