mtd: fsl-quadspi: Remove unneeded driver.bus assignment

platform_driver_register() takes care of assigning driver->bus
to &platform_bus_type, no need to explicitly assign it in the driver.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>