wifi: mac80211: fix puncturing bitmap handling in CSA

Code inspection reveals that we switch the puncturing bitmap
before the real channel switch, since that happens only in
the second round of the worker after the channel context is
switched by ieee80211_link_use_reserved_context().

Fixes: 2cc25e4b ("wifi: mac80211: configure puncturing bitmap")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>