synclink_gt: fix transmit race and timeout

Fix race condition when adding transmit data to active DMA buffer ring
that can cause transmit stall.

Update transmit timeout when adding data to active DMA buffer ring.
Base transmit timeout on amount of buffered data instead of using fixed
value.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>