atmel_serial: fix flow control bug

Fix the following problem, related to hardware flow control (CTS/RTS):
Transmitting while CTS line is asserted in DMA mode, due to not checking
for tx-stopped condition.

We found these problems while testing the UARTs with hardware
flow-control.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: "Andrew Victor" <avictor.za@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>