mac80211: set channel only once during auth/assoc

mac80211: set channel only once during auth/assoc

There's no need to set up the channel during auth
and again during assoc, just do it once. Currently
this doesn't result in any changes since calling
hw_config() with an unchanged channel will return
early, but with the channel context work this has
an impact on channel context assignment.

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