ethdev: add doxygen comments for each state

ethdev: add doxygen comments for each state

The enum rte_eth_dev_state was not properly documented.
Its values did not appear in the doxygen output,
and may be misunderstood.

The state RTE_ETH_DEV_DEFERRED has no interest anymore
since the ownership mechanism brings a more flexible categorization.
This state could be removed later.

Fixes: d52268a8b24b ("ethdev: expose device states")
Fixes: cb894d99eceb ("ethdev: add deferred intermediate device state")
Fixes: 5b7ba31148a8 ("ethdev: add port ownership")
Fixes: 7106edc12380 ("ethdev: add devop to check removal status")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Matan Azrad <matan@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>