regulator: mc13892: remove the unnecessary prefix from regulator name

It's not really necessary to add a prefix 'MC13892__' for each mc13892
regulator name, since the chip must have been identified as mc13892
when we look at the regulator name.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>