can: kvaser_usb: Fix sparse warning __le16 degrades to integer

USB endpoint's wMaxPacketSize field is an le16 entity. Use
appropriate le16_to_cpu macros to maintain endian independence.

Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Ahmed S. Darwish <ahmed.darwish@valeo.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>