cfg80211/mac80211: re-add get_channel operation

This essentially reverts commit 2e165b81

 but
introduces the get_channel operation with a new
wireless_dev argument so that you can retrieve
the channel per interface. This is necessary as
even though we can track all interface channels
(except monitor) we can't track the channel type
used.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>