mfd: ab8500-sysctrl: Set sysctrl_dev during probe

The driver requires sysctrl_dev to be set at probe, as it's used by
other driver functions.  This was dropped by mistake in:

2377e52f mfd: ab8500-sysctrl: Error check clean up

making all driver functions fail.

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