[Blackfin] arch: fix bug - kgdb causing kernel panic

ignore soft bp except when single step except occurs.
Narrow the condition to ignore soft bp.
If soft bp occurs in the single step handler or the soft bp handler,
return immediately. This could happen when gdb client set soft bps in
kernel code that in the path of kgdb core code.

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