mtd: brcmnand: defer to devm_ioremap_resource() for error checking

devm_ioremap_resource() does error checking on the 'res' argument, so
drop the error check in bcm6368_nand.c.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Tested-by: Simon Arlott <simon@fire.lp0.eu>