ethdev: do not rely on detachable flag in detach

This flag is deprecated and should not be used to check for the device
ability to be detached.

The rte_dev library call will fail with the relevant error code if
detaching this port is not possible.

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>