disable ipv6 for broadcom SONiC fanout devices. (sonic-net#21956)

Description of PR
Summary:
Fixes test_qos_sai failures due to unexpected ipv6 packets coming from fanout device

Type of change
 Bug fix
 Testbed and Framework(new/improvement)
 New Test case
 Skipped for non-supported platforms
 Test case improvement

Approach
What is the motivation for this PR?
Disable ipv6 for broadcom SONiC fanout devices.

How did you do it?
Add broadcom into the list.

How did you verify/test it?
physical testbed.

Signed-off-by: Dashuai Zhang <dashuaizhang@microsoft.com>
Signed-off-by: Saravanan Sellappa <saravanan@nexthop.ai>

Author: sdszhang

ansible/roles/fanout/tasks/sonic/fanout_sonic_202505.yml

@@ -181,7 +181,7 @@
     shell: "sysctl -p"
     become: yes
 
-  when: "fanout_sonic_version.asic_type in ['marvell-teralynx', 'mellanox']"
+  when: "fanout_sonic_version.asic_type in ['marvell-teralynx', 'mellanox', 'broadcom']"
 
 - name: Stop lldp container
   block: