resulttool/regression.py: fix AttributeError

resulttool/regression.py: fix AttributeError

Fix following AttributeError when running "resulttool regression base target":
  File "/yocto/poky/scripts/lib/resulttool/regression.py", line 322, in regression_common
    res, resstr = compare_result(logger, c, b, base_results[a][c], target_results[a][b], args.limit)
AttributeError: 'Namespace' object has no attribute 'limit'

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>