staging: brcm80211: fix compiler warning introduced

Patch "[0f06b6a] remove WLC_WATCHDOG_TBTT macro" resulted
in compiler warning because static function wlc_watchdog_backup_bi()
is no longer used. This has been fixed in this patch.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>