[net80211] Quieten the logging from ieee80211_vht_get_vhtflags()

The commit in Fixes: introduced logging the output bits from
ieee80211_vht_get_vhtflags(). This ends up causing quite a lot
of logging when net80211 is doing things like processing
received beacons.

So just remove the logging; if it's needed again then a developer
can add it back to that location, or just use dtrace to capture
the return value.

Fixes: 4bf049bfeefd9
Differential Revision: https://reviews.freebsd.org/D52142
Reviewed by: bz