regulator: Staticise mc13783_powermisc_rmw()

It is not used outside this driver so no need to make the symbol global.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Alberto Panizzo <maramaopercheseimorto@gmail.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>