addons: mstpctl: call disable_ipv6 only if mstpctl_ports is specified

addons: mstpctl: call disable_ipv6 only if mstpctl_ports is specified

Ticket:
Reviewed By: trivial
Testing Done: tested sanity and bridge bringup and reload

bridge module already disables ipv6 on ports when bridge-ports is
specified. Making this change because i saw redundant calls to
disable_ipv6 on bridge ports by modules bridge and mstpctl
when bridge-ports is specified. It is slowing down ifreload

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>