pinctrl: nomadik: silence uninitialized variable warning

This is harmless, but "val" isn't necessarily initialized if
abx500_get_register_interruptible() fails.  I've re-arranged the code to
just return an error code in that situation.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>