staging: rtl8723au: struct wlan_bssid_ex: Rename BeaconPeriod to beacon_interval

This is to be more consistent mapping the names in wlan_bssid_ex to
those in struct ieee80211_mgmt.

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