usbip: usbip_detach: fix to check for invalid ports

usbip detach doesn't check for invalid ports and ports that are already
detached. It attempts to remove state file(s) without validating the port
and sends detach request to the driver for ports that are already detached.

Add check for invalid ports (port > maxports) and ports that are already
detached (status == VDEV_ST_NULL). Don't remove state files and don't send
detach request for invalid ports and ports that are already detached.

Add error and information messages that make sense.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>