mtd: check for valid pdata inside plat_nand

If plat_nand loads and the platform_data is not properly set it will segfault.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>