can: gs_usb: fixed disconnect bug by removing erroneous use of kfree()

gs_destroy_candev() erroneously calls kfree() on a struct gs_can *, which is
allocated through alloc_candev() and should instead be freed using
free_candev() alone.

The inappropriate use of kfree() causes the kernel to hang when
gs_destroy_candev() is called.

Only the struct gs_usb * which is allocated through kzalloc() should be freed
using kfree() when the device is disconnected.

Signed-off-by: Maximilian Schneider <max@schneidersoft.net>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>