serial: mxs-auart: unmap the scatter list before we copy the data

We should first unmap the DMA scatter list for receiving data, and
then copy the data from the DMA buffer.

The old code misses unmap the scatter list for RX. This patch fixes it.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Tested-by: Lauri Hintsala <lauri.hintsala@bluegiga.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>