mac80211: fix order of setting ch_switch and drv_pre_channel_switch call

There was a mistake when merging commit 6d027bcc (mac80211: add
pre_channel_switch driver operation) for upstream.  The assignment of
the values in the ch_switch structure came below the call to
drv_pre_channel_switch.  Fix the order.

Fixes: 6d027bcc (mac80211: add pre_channel_switch driver operation)
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>