spi: pl022: handle EPROBE_DEFER for dma

Handle EPROBE_DEFER explicitly so that we ensure that we get the DMA
channel specified in the device tree, instead of depending on the DMA
controller getting probed before us.

Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Signed-off-by: Mark Brown <broonie@kernel.org>