can: esd_usb2: fix memory leak on disconnect

It seems struct esd_usb2 dev is not deallocated on disconnect. The patch adds
the missing deallocation.

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

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>