USB: serial: set driver owner when registering drivers

Modules registering drivers with usb_serial_register_drivers() might
forget to set .owner field.  The field is used by some of other kernel
parts for reference counting (try_module_get()), so it is expected that
drivers will set it.

Solve the problem by moving this task away from the drivers to the core
USB serial code, just like we did for platform_driver in
commit 9447057e ("platform_device: use a macro instead of
platform_driver_register").

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
[ johan: amend commit summary ]
Signed-off-by: Johan Hovold <johan@kernel.org>