iwlwifi.4: add link to bugzilla in the BUGS section

Given iwlwifi has a (hopefully) fully categorized bug-set under one
meta-bug (tracking) we can give a link to that.

Suggested by: grahamperrin (as part of a large GH pull request)
Suggested by: ziaee (final markup)
Sponosred by: The FreeBSD Foundation
Reviewed by: ziaee
Pull Request: https://github.com/freebsd/freebsd-src/pull/1541