selftests: usbip: add wait after attach and before checking port status

Add sleep between attach and "usbip port" check to make sure status is
updated. Running attach and query back shows incorrect status.

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