[Blackfin] arch: change the trace buffer control start/stop logic in the exception handlers

To save/restore the trace buffer control so that if we take an exception
after turning off the trace buffer at a higher level we dont inadvertently
turn the trace buffer back on

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>