If "full offload scan" is set, we do not want to continue or start
a background scan (the firmware will do that anyway by its own leisure).
Simplify the logic and pull that case out upfront.
This is assumed to help iwm(4), LinuxKPI based drivers supporting hw_scan
and rsu(4) to not get into a state where a scan is always assumed to be
running but no scan is actually active.
While where fix spelling and use an != 0 check for style on a
conition check for IEEE80211_FEXT_SCAN_OFFLOAD in sta_roam_check().
Sponsored by: The FreeBSD Foundation
MFC after: 3 days