ena: Update driver to use ethtool_sprintf

Replace instances of snprintf or memcpy with a pointer update with
ethtool_sprintf.

Acked-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>