mfd: Prevent unassigned pointer from being used in ab8500-gpadc driver

Before this patch if probe failed to find the platform IRQ it
would attempt to print a message out using dev_err, which in
turn was being passed an unassigned pointer. This patch
ensures the information passed to dev_err is correct.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>