serial: imx: remove the redundant code

In the imx_startup(), we will reset the uart port which will reset all
the FIFOs, including the URXD.

So the code to clear the RX FIFO is redundant. Just remove it.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>