serial: msm_geni_serial: Resolve race btw stop rx and DMA rx

Resolve potential race btw stop rx and DMA rx due to interrupt latency.

Change-Id: I3f9587390f7b471c5486b72c8c76fbd83b6c616b
Signed-off-by: Akash Asthana <akashast@codeaurora.org>