mtd: spi-nor: cadence-quadspi: Don't initialize rx_dma_complete on failure

If driver fails to acquire DMA channel then don't initialize
rx_dma_complete struct as it won't be used.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20200601070444.16923-4-vigneshr@ti.com


Signed-off-by: Mark Brown <broonie@kernel.org>