spi: spi-geni-qcom: Check for Rx DMA channel being null

If Rx dma request channel fails, log it as an error and
exit the gsi event callback setup routine.

Change-Id: Idb92be41ddce2011eb0f74674cb31150860ae9bc
Signed-off-by: Vipin Deep Kaur <vkaur@codeaurora.org>