ethdev: avoid explicit check of valid port state

ethdev: avoid explicit check of valid port state

Some port iterations are manually checking against RTE_ETH_DEV_UNUSED
instead of using the iterators based on rte_eth_find_next().

A new macro RTE_ETH_FOREACH_VALID_DEV() is introduced, but kept private
because there should be no need of iterating over all devices in the
API. The public iterators have additional filters for ownership, parent
device or sibling ports.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>