mfd: AB3100 propagate error

This makes ab3100_set_register_interruptible() propagate the error
code from suboperations properly so it can be handles properly.
(A special case comes from signal interruption.)

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