[net80211] Don't check bgscanidle setting in net80211 for full-offload scan.

If full-offload scan is used, the NIC driver (or rather the firmware of
the NIC) should take care of interrupting and continuing the background
scan. So net80211 should ignore the vap->iv_bgscanidle setting then, instead
the NIC driver might look at this setting and pass it on to the firmware
in some way if possible.

Since full-offload scans won't be explicitly interrupted by net80211, it
also doesn't really make sense to check the vap->iv_bgscanidle condition
in that case, before starting a background scan. If the NIC driver
advertises background scan support and full-offload scanning, the firmware
should be able to execute that scan without interfering too much with our
data traffic.

Reviewed by: adrian, avos
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D8539