[Blackfin] arch: fix bug - before assign new channel to the map register, need...

[Blackfin] arch: fix bug - before assign new channel to the map register, need clear the bits first.

http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=2445



Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>