synclink_gt: fix transmit DMA stall

Fix transmit DMA stall when write() called in window after previous
transmit DMA completes but before previous serial transmission completes.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>