pim6d: Don't display MLD disabled or down interfaces in "show ipv6 mld interface...

pim6d: Don't display MLD disabled or down interfaces in "show ipv6 mld interface" cmd

We should not display down interfaces or MLD disabled interfaces in
"show ipv6 mld interface" command.

Before fix:
==========
frr# show ipv6 mld interface
Interface         State  V  Querier                    Timer               Uptime
ens192            up     2  fe80::250:56ff:feb7:d04    query 00:00:25.432  00:00:07.038
ens224            up     1  fe80::250:56ff:feb7:a7e3   query 00:00:24.219  00:00:07.031
pim6reg            down

After fix:
=========
frr# show ipv6 mld interface
Interface         State  V  Querier                    Timer               Uptime
ens192            up     2  fe80::250:56ff:feb7:d04    query 00:00:25.432  00:00:07.038
ens224            up     1  fe80::250:56ff:feb7:a7e3   query 00:00:24.219  00:00:07.031

Issue: #11241

Signed-off-by: Sarita Patra <saritap@vmware.com>