mfd: ab8500-sysctrl: Error check clean up

Add error checks to see if sysctrl was probed as it should.
If the sysctrl_dev is not set the return value is -EINVAL.

Signed-off-by: Marcus Danielsson <marcus.danielsson@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Mattias WALLIN <mattias.wallin@stericsson.com>
Tested-by: Per FORLIN <per.forlin@stericsson.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>