serial: msm_geni_serial: Pull RFR before stop_rx for uart

Pull the RFR line before doing stop_rx in uart driver.
This is to prevent any rx data to come into the uart rx fifo when
stop_rx is being executed.
Remove the rx length zero check in
handling dma rx to prevent race conditions with EOT and DMA_DONE
interrupt bits.

Issue scenarios could be that during baud rate change/ runtime
suspend or any other place where stop_rx is used, there might
be a race between rx data entering uart rx fifo and availability
of dma buffers in uart driver. In this scenarios there will
be interrupt storm with rx water mark interrupt.

Change-Id: I92163c4997c1709269dce67a1d492533f710abfb
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>