mtd: fsl-quadspi: fix __iomem annotations

This corrects some sparse warnings:

   drivers/mtd/spi-nor/fsl-quadspi.c:281:31: warning: incorrect type in initializer (different address spaces) [sparse]
   drivers/mtd/spi-nor/fsl-quadspi.c:281:31:    expected void *[noderef] <asn:2>base [sparse]
   drivers/mtd/spi-nor/fsl-quadspi.c:281:31:    got void [noderef] <asn:2>*iobase [sparse]
   (etc.)

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Huang Shijie <b32955@freescale.com>