serial: msm_geni_serial: set unmapped rx dma buffer to NULL

Set the Rx buffer to NULL post un-mapping it from the iommu as part
of ISR handling. Otherwise the buffer pointer being non-NULL would
cause unmapped access while stop_rx gets called.

Change-Id: I9f2e89ea36e723d93d524bd46827c6ca4395e790
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>