mfd: Don't use mutex_lock_interruptible in ab8500-core

This disadvantage of having it interruptible is that it would
appear to work OK in most situations, but in the rare case
that this call does fail, it can lead to misconfiguration
of the AB, such as a regulator not being turned off and
leading to a platform power consumption increase.

There's no real benefit to having this interruptible.

Make it a plain mutex_lock. The non-interruptability matches
other low-level I/O functions such as SPI and I2C.

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