mac80211: restrict advertised HW scan rates

Advertise only user-requested bitrates in a HW scan.
Note that the hw_scan API doesn't currently have a
way of asking for a specific probe request bitrate,
so we might end up using a bitrate that we don't
advertise as supported. I'll fix that later.

Also add a hexdump printk to hwsim to verify this.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>