[Blackfin] serial driver: Fix bug Free rx dma buffer in shutdown.

Kernel crash for the serial driver in DMA mode:
http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3679



Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>