mtd: fsl-quadspi: possible NULL dereference

It is theoretically possible to probe this driver without a matching
device tree, so let's guard against this.

Also, use the of_device_get_match_data() helper to make this a bit
simpler.

Coverity complained about this one.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Han xu <han.xu@freescale.com>