LinuxKPI: 802.11 correct enum ieee80211_channel_flags

enum ieee80211_channel_flags are used as bit fields and not as 1..n.
Correct the values using BIT(n).

This is also hoped to fix problems with 7260 cards which come up and
panic due to an empty channel list as all channels are set disabled [1].
It will hopefully also fix the one or other oddity.

Reported by: ambrisko, Mike Tancsa (mike sentex.net) [1]
Sponsored by: The FreeBSD Foundation
MFC after: 3 days