[Blackfin] arch: Don't oops_in_progress if single step is comming from the kernel

Don't oops_in_progress if single step is comming from the
kernel, which happens if a single step occurs after a exception cause.
This fixes up the remaining issues in the toolchain bug.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>