mfd: Refactor db8500-prcmu request_clock() function

This refactors the mfd/dbx500-prcmu drivers to use a switch()
statement rather than nested if/else-construction.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>