USB: console: add dummy __module_get

Add call to __module_get when initialising the fake tty in
usb_console_setup to match the module_put in release_one_tty.

Note that the tty-driver (i.e. usb-serial core) must be compiled-in to
enable the usb console so the __module_get is essentially a noop as
driver->owner will be null.

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Johan Hovold <johan@kernel.org>