serial: msm_geni_serial: Make sure to have buffer for stop rx

This patch creates RX buffer required to handle EOT event as part
of stop rx operation. In some rare scenario it may happen that stop rx
may not find the DMA buffer if there is some unexpected sequence gets
executed for any reason.

Change-Id: Iaf74170ca71b4247b46af2e69a8a19ccf4166cdf
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>