Use the provided interface in attach_filter()

This is related to the e-mail thread[1] on Scapy mailing list, based
on the proposed PR on Bitbucker Suraj Deshmukh [2].

[1] http://article.gmane.org/gmane.comp.security.scapy.general/5297
[2] https://bitbucket.org/secdev/scapy/pull-requests/127/issue-5121-now-iface-option-of-sniff/diff