ems_usb: Fix byte order issues on big endian machines

CPC-USB is using a ARM7 core with little endian byte order. The "id" field
in can_msg needs byte order conversion from/to CPU byte order.

Signed-off-by: Sebastian Haas <haas@ems-wuensche.com>
Signed-off-by: David S. Miller <davem@davemloft.net>