ovsdb: set DPDK port MTU when creating them

The DPDK port will not have a link after the devbind which is needed for
configuring an interface to be a DPDK port. The MTU is being committed
during the link change but for DPDK ports there is no link.

The DPDK port MTU should be set on ovsdb right after the interface is
added to ovsdb. This way the users will be able to set MTU for DPDK
ports and modify it.

Please see the following results:
  port 2: iface0 (dpdk: configured_rx_queues=1, configured_rxq_descriptors=2048, configured_tx_queues=3,
configured_txq_descriptors=2048, lsc_interrupt_mode=false, mtu=2000, requested_rx_queues=1,
requested_rxq_descriptors=2048, requested_tx_queues=3, requested_txq_descriptors=2048, rx_csum_offload=true, tx_tso_offload=false)
57 jobs for ff/dpdk_ovs_mtu in 33 minutes and 5 seconds (queued for 18 seconds)