USB: serial: metro-usb: fix up usb_serial_register calls

The usb serial core has changed how the driver is to be registered and
unregistered recently.  Make these changes to the driver so that it will
properly build and work.

Cc: Aleksey Babahin <tamerlan311@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>