Wait for endptmngr during chan_brcm module load

There was a situation that during asterisk restart -> chan_brcm.so
module loading, endptmngr was not up and running yet, causing the ubus
lookup for endpt_id failed. It caused chan_brcm.so module not loaded
and asterisk not fully working properly. Add waiting for endptmngr fixes
the issue.

Signed-off-by: Grzegorz Sluja <grzegorz.sluja@iopsys.eu>