app/testpmd: check not detaching device twice

app/testpmd: check not detaching device twice

The command "port detach" is removing the EAL rte_device
of the ethdev port specified as parameter.
The function name and some comments are updated to make clear
that we are detaching the whole device.

After detaching, the pointer, which maps a port to its device,
is reset. This way, it is possible to check whether a port
is still associated to a (not removed) device.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>