mac80211: use correct address for mesh Path Error

For forwarded frames, we save the precursor address in addr1 in case it
needs to be used to send a Path Error.  mesh_path_discard_frame,
however, was using addr2 instead of addr1 to send Path Error frames, so
correct that and also make the comment regarding this more clear.

Signed-off-by: Andrey Yurovsky <andrey@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>