app/testpmd: add port check before manual detach

app/testpmd: add port check before manual detach

User may try to run "port detach <port_id>"
for an already detached device.
It has been decided to protect from such usage in testpmd,
so a check was added to detach_port_device() in DPDK 19.11.
This check might be removed to allow hotplug path detaching
the device of a closed port.
Whatever will be decided in future, this check is also added
before the call to detach_port_device().

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>