ionic: Fix check in ionic_get_link_ext_stats

ionic: Fix check in ionic_get_link_ext_stats

The current check will fail if SR-IOV is not initialized for the
physical function; this is because is_physfn is 0 if sriov_init() isn't
run or fails. Change the check that prevents getting the link down count
to use is_virtfn instead so that VFs don't get this functionality, which
was the original intent.

Fixes: 132b4ebfa090 ("ionic: add support for ethtool extended stat link_down_count")
Signed-off-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Eric Joyner <eric.joyner@amd.com>
Link: https://patch.msgid.link/20260614205303.48088-2-eric.joyner@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>