staging: rtl8723au: Update bss beacon info in rtw_add_beacon()

Pull out the core info beacon_interval, capability, and tsf and update
cur_network.network with the info in rtw_add_beacon() instead of
relying on it being in ->IEs.

This will help later when getting rid of the beacon struct info from
->IEs and only carrying the actual IEs there.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>