can: ems_usb: ems_usb_start_xmit(): fix double dev_kfree_skb() in error path

There is no need to call dev_kfree_skb() when usb_submit_urb() fails
beacause can_put_echo_skb() deletes the original skb and
can_free_echo_skb() deletes the cloned skb.

Link: https://lore.kernel.org/all/20220228083639.38183-1-hbh25y@gmail.com
Fixes: 702171ad

 ("ems_usb: Added support for EMS CPC-USB/ARM7 CAN/USB interface")
Cc: stable@vger.kernel.org
Cc: Sebastian Haas <haas@ems-wuensche.com>
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>