serial: imx: start rx_dma once RXFIFO is not empty

Start rx_dma once RXFIFO is not empty that can avoid dma request lost
and causes data delay issue.

Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>