ethdev: do nothing if promiscuous mode is applied again

Since driver callbacks return status code now, there is no necessity
to enable or disable promiscuous mode once again if it is already
successfully enabled or disabled.

Configuration restore at startup tries to ensure that configured
promiscuous mode is applied and start will return error if it fails.

Also it avoids theoretical cases when already configured promiscuous
mode is applied once again and fails. In this cases it is unclear
which value should be reported on get (configured or opposite).

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>