netdev-dpdk: Disable NIC offloading and multiseg mbufs

netdev-dpdk: Disable NIC offloading and multiseg mbufs

We do not use any offloading (now) or multiple segments per packet, so
we might as well disable those features while configuring the NIC.

This could give performance improvements. For ixgbe, for example, this change
allows the driver to use a simpler tx routine, resulting in throuput
improvements (~7.5%)

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Thomas Graf <tgraf@noironetworks.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>