ring: enhance device setup from rings

ring: enhance device setup from rings

The ring ethdev creation function creates an ethdev, but does not
actually set it up for use. Even if it's just a single ring, the user
still needs to create a mempool, call rte_eth_dev_configure, then call
rx and tx setup functions before the ethdev can be used.

This patch changes things so that the ethdev is fully set up after the
call to create the ethdev. The above-mentionned functions can still be
called - as will be the case, for instance, if the NIC is created via
commandline parameters - but they no longer are essential.

The function now also sets rte_errno appropriately on error, so the
caller can get a better indication of why a call may have failed.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>