Blackfin Serial Driver: fix bug - use mod_timer to replace only add_timer.

http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=4045
If adding timer in both timer handler and rx interrupt, a timer may be
added when it is till in the pending list.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>