[Blackfin] serial driver: Fix bug serial driver in DMA mode spams history to console on shell restart

http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=2920
Fix by increasing buffer tail immediately before starting tx dma.
Signed-off-by: NSonic Zhang <sonic.zhang@analog.com>
Signed-off-by: NBryan Wu <bryan.wu@analog.com>