serial: imx: unmap scatter gather list in imx_flush_buffer

First, reformat the code to exit immediately. This allows us to add
more code in more readable format.

In case the TX DMA was still running, remove and disable it's resources.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>