mfd: AB3100 accessor function cleanups

This adds the _interruptible suffix to the AB3100 accessor
functions on par with mutex_lock_interruptible() that's used
for blocking simultaneous calls to the AB3100 acessor functions.
Since these accesses are slow on a 100kHz I2C bus and may line
up waiting for the mutex, we need to handle interruption by
system shutdown or kill signals and may just as well denote that
in the function names.

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