mtd: lpc32xx_slc: Make probe() return -EPROBE_DEFER if necessary

Via of_get_named_gpio(), wp_gpio can become -EPROBE_DEFER which now makes
probe() return -EPROBE_DEFER as well to wait until the gpio controller is
probed before trying to probe lpc32xx_slc again.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>