ipfw: do not use errno value for error reporting

table_do_modify_record() already uses errno value on error.
Also this fixes problem when ipfw table add returns ENOTTY that
is unrelated to operation.

Tested by: dhw
Fixes: 09025a714708
Differential Revision: https://reviews.freebsd.org/D53050

(cherry picked from commit dfd822b1f0846bdaa9a14457346f7431a86d3a64)