serial: msm_geni_serial: Correct start rx sequence

Correct start_rx sequence to keep the dma buffers available
before starting rx engine.
This is needed to handle rx data which might come between
starting of rx engine and having dma buffer available.

Change-Id: I194890ba8f51d6a917d6b1682f6b21ca443ca38b
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>