can: peak_usb: fix multi-byte values endianess

This patch fixes the endianess definition as well as the usage of the
multi-byte fields in the data structures exchanged with the PEAK-System USB
adapters.

By fixing the endianess, this patch also fixes the wrong usage of a 32-bits
local variable for handling the error status 16-bits field, in function
pcan_usb_pro_handle_error().

Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>