ethdev: clarify API of packet types support
ethdev: clarify API of packet types support

As discussed in http://dpdk.org/ml/archives/dev/2016-June/042229.html,
clarify the behavior of rte_eth_dev_get_supported_ptypes().

Signed-off-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk