Fix a blatant misuse of MINCLSIZE I introduced in ieee80211_output.c r1.59

Fix a blatant misuse of MINCLSIZE I introduced in ieee80211_output.c r1.59
The bug was added on 04/16 but it started to show up only after 06/12 when
the value of MINCLSIZE was changed to something different from MHLEN + 1.

Thanks to dlg@ and giovanni (qgiovanni at gmail dot com) for putting me
on the right track.

Tested by giovanni.
Should fix system/5881 too.