spi: spi-ti-qspi: Use dma_engine wrapper for dma memcpy call

Instead of calling device_prep_dma_memcpy() directly with dma_device
pointer, use the newly introduced dmaengine_prep_dma_memcpy() wrapper
API.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>