LinuxKPI: 80211: scanning code updates

For consistency rename LKPI_SCAN_RUNNING adding a LHW_ prefix.
Add a local flag LKPI_LHW_SCAN_HW mirroring the net80211
IEEE80211_FEXT_SCAN_OFFLOAD flag.
Slightly simplify the code.
Overload (*ic_scan_curchan) and (*ic_scan_mindwell) so that we can
call the net80211 implementation in case of software scan but skip it
in case of full-offload scans.
Also add a bandaid to our (*ic_set_channel) implementation to not siwtch
channels if we have an active hw_scan running.

Obtained from: bz/wireless-dev
Sponsored by: The FreeBSD Foundation (partially)

(cherry picked from commit a486fbbd7876bed81d738a32274953c89906edb5)