can: ems_usb: Fix possible tx overflow

This patch fixes the problem that more CAN messages could be sent to the
interface as could be send on the CAN bus. This was more likely for slow baud
rates. The sleeping _start_xmit was woken up in the _write_bulk_callback. Under
heavy TX load this produced another bulk transfer without checking the
free_slots variable and hence caused the overflow in the interface.

Signed-off-by: Gerhard Uttenthaler <uttenthaler@ems-wuensche.com>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>