ethdev: check device promiscuous state

ethdev: check device promiscuous state

The promiscuous enable and disable functions now check the
promiscuous state of the device before checking if the dev_ops
function exists for the device.

This change is necessary to allow sample applications run on
virtual PMDs, as previously -ENOTSUP returned when the promiscuous
enable function was called. This caused the sample application to
fail unnecessarily.

Signed-off-by: Ciara Power <ciara.power@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>