net/mlx: support firmware version query

net/mlx: support firmware version query

The API function rte_eth_dev_fw_version_get() is querying drivers
via the operation callback fw_version_get().
The implementation of this operation is added for mlx4 and mlx5.
Both functions are copying the same ibverbs field fw_ver
which is retrieved when calling ibv_query_device[_ex]()
during the port probing.

It is tested with command "drvinfo" of examples/ethtool/.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>