can: ems_usb: mark timestamp as little endian

The struct ems_cpc_msg describes the a message received from the USB device,
which uses little endian byte order. This patch marks the timestamp in struct
ems_cpc_msg accordingly.

Acked-by: Gerhard Uttenthaler <uttenthaler@ems-wuensche.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>