mac80211: pass channel to ieee80211_send_probe_req

In multi-channel scenarios, the channel that we will
transmit a probe request on isn't always the current
channel (which will be NULL anyway) but will instead
be the channel that the AP is on. Pass the channel
to the ieee80211_send_probe_req() function so it can
be used in the different scenarios. The scan code
continues to pass the current channel, of course.

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