app/testpmd: fix wrong API of adding VF MAC

When adding a VF MAC address, rte_eth_dev_mac_addr_add is called.
It's not right, because this API is used to add a MAC address for
a VMDq pool not a VF. Although it can work on ixgbe as VMDq pool
and VF mean the same thing on ixgbe.

Fixes: 7741e4cf16c0 ("app/testpmd: VMDq and DCB updates")

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>