USB: zte_ev: fix broken open

Remove bogus port-number check in open and close, which prevented this
driver from being used with a minor number different from zero.

Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>