Blackfin: fix bug found by traps test case 21

The traps test case 21 "exception 0x3f: l1_instruction_access" would make
the kernel panic on BF533's because we end up calling show_stack()
infinitely.

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