serial: imx: fix the wrong number of scatterlist entries when xmit->head is 0

When the (xmit->tail > xmit->head) is true and the xmit->head is 0,
we only need one DMA scatterlist in actually. Current code uses two DMA
scatterlists in this case, this is obviously wrong.

This patch fixes it.

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