USB: serial: option: add support for Quectel EM12

The Quectel EM12 is a Cat. 12 LTE modem. It behaves in the exactly the
same way as the EP06 (including the dynamic configuration behavior), so
the same checks on reserved interfaces, etc. are needed.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>