af_packet: make the device detachable

af_packet: make the device detachable

Allow dynamic deallocation of af_packet device through proper
API functions. To achieve this:
* set device flag to RTE_ETH_DEV_DETACHABLE
* implement rte_pmd_af_packet_devuninit() and expose it
  through rte_driver.uninit()
* copy device name to ethdev->data to make discoverable with
  rte_eth_dev_allocated()
Moreover, make af_packet init function static, as there is no
reason to keep it public.

Signed-off-by: Wojciech Zmuda <woz@semihalf.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>