nand: mrvl: use ERR_CAST() for returning error pointers
ERR_CAST exists to return error pointers as error pointers without
casting them explicitly to the correct pointer type.
Also this Fixes:
In function 'alloc_nand_resource':
warning: return makes pointer from integer without a cast
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>