mfd: db8500-prcmu: Return early if the TCPM cannot be located

Currently we check to see if we obtained the Tightly Coupled Program
Memory (TCPM) base and only execute the code within the check if we
have it. It's more traditional to return early if we don't have it.
This way we can flatten most of the function's code down to a single
tab spacing.

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