Blackfin arch: Fix bug - hardware breakpoint doesn't always work in kgdb

Hardware breakpoint doesn't always work in kgdb. It works at the first
two times, but if you repeatedly trigger that hardware breakpoint, it
would slip over that point once in two times.

Fix it by always setting hw bp skip to 0. gdb does skip after hw bp trap.

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