spi: spi-msm-geni: Propagate missing change from 4.19 to 5.4

Propagate following change from 4.19 to 5.4 for SPI driver.
'commit 64485a92b029 ("spi: spi-geni-qcom: Check for Rx DMA channel
being null")'.

Change-Id: I0dc30e38d0cbbab21d26eeab17b40b14017d48ff
Signed-off-by: Vipin Deep Kaur <vkaur@codeaurora.org>