mtd: nand-gpio: don't waste memory for OF failure

We shouldn't try to allocate a resource until we're sure the
of_property_read_u64() call didn't fail. This is especially important if
we use this code for both CONFIG_OF and !CONFIG_OF builds, since
of_property_read_u64() will always return -ENOSYS for !CONFIG_OF.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>