Use mc13xxx_reg_rmw rather than a mc13xxx_reg_read and a mc13xxx_reg_write calls.

This logic to set MC13892_SWITCHERS0_SWxHI bit is pretty simple:

if (volt > 1375000)
        set MC13892_SWITCHERS0_SWxHI bit
else if (volt < 1100000)
        clear MC13892_SWITCHERS0_SWxHI bit
else
        leave MC13892_SWITCHERS0_SWxHI bit untouched

We already know the selector, so we don't need to calculate the selector again.
Signed-off-by: NAxel Lin <axel.lin@gmail.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>