backport of r46749

Without this patch adding the wifi device to the bridge may fail
because the bridge is not already configured when the wifi device gets
configured. This patch makes broadcom-wl wait till the bridge is ready.

This fixes #17262
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>


git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46750 3c298f89-4303-0410-b956-a3cf2f4a3e73