Input: gtco - fix usb_dev leak

There is usb_get_dev() in gtco_probe(), but there is no usb_put_dev()
anywhere in the driver.

As pointed out by Dmitry Torokhov:
The lifetime of gtco structure is already directly tied to lifetime of
usb_dev: when destroying usb_dev driver core will call remove() function
of currently bound driver (in our case gtco) which will destroy gtco
memory. Taking additional reference is not needed here.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>