app/testpmd: fix invalid port detach

Using registration to all ports includes also the ports which should not
be used by the application.

It is nice to print each port event by testpmd but in case of RMV
event, testpmd tries to detach the port and this case is problematic
when the port should not be used by the application.

Check the port validation before detach API calling.

Fixes: 4fb82244b394 ("app/testpmd: extend event printing")

Signed-off-by: Matan Azrad <matan@mellanox.com>