staging: rtl8723au: collect_bss_info23a(): Collect beacon_interval correct

Only retrieve beacon_interval from frames where it is present. This
avoids setting BeaconPeriod to garbage for other frame types.

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