LinuxKPI: 802.11: cleanup chanctx freq/width settings

Cleanup chanctx freq1/2 and width settings using the channel from the
net80211 node and the net80211 functions to return the center
frequencies (already doing the math for us or using pre-computed values).

Set the min_def (minimum channel definition) to def (channel definition)
for the moment as it seems to be unused by anything but
iwl_mvm_chanctx_def() to avoid complications.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days