Corrected the read_mcast_stats function, to display
"ubus call mcast stats" correct output for multiple iptv bridges
Test:
"ubus call mcast stats" showing correct output for both single and multiple bridge.
Script for generating mcast stats is moved from common to platform
specific files. The logic for generating mcast stats on non broadcom
platforms is different and the stats format is also different.
For generating stats we are are sending SIGUSR1 signal to mcproxy
process which then writes multicast stats to /tmp/igmp_snooping_stats.
