Blackfin arch: fix bug - crashes in tcp_v4_send_reset

Michael says that some bugs are crashes in tcp_v4_send_reset.
There's a missing clobber of "CC" in our checksum assembly
statement; fixing this makes the generated code look much saner.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>