Change share_vif_oper_freq() to handle multiple freqs

Change share_vif_oper_freq() to handle multiple freqs

There are devices that can operate several channels concurrently.
Change shared_vif_oper_freq() to get_shared_radio_freqs() that can
return an array of frequencies currently used by all the virtual
interfaces that share the same radio.

In addition, move it to wpa_supplicant.c, so it can be used by other
modules.

Signed-hostap: Ilan Peer <ilan.peer@intel.com>
Signed-hostap: David Spinadel <david.spinadel@intel.com>