spi: spi-cadence-quadspi: Fix mapping of buffers for DMA reads

Buffers need to mapped to DMA channel's device pointer instead of SPI
controller's device pointer as its system DMA that actually does data
transfer.
Data inconsistencies have been reported when reading from flash
without this fix.

Fixes: 5b6fc55d ("spi: Move cadence-quadspi driver to drivers/spi/")
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>