LinuxKPI: 802.11: check IEEE80211_FEXT_SCAN_OFFLOAD on vap

When checking for IEEE80211_FEXT_SCAN_OFFLOAD, do so on the vap rather
than ic. This brings us in line with what net80211 does.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 768dd32a164b35fb2f66fc8a0250cee84c79471e)