can: peak_usb: pcan_usb: replace open coded endianness conversion of unaligned data

This patch replaces the open coded endianness conversion of unaligned
data by the appropriate get/put_unaligned_leXX() variants.

Link: https://lore.kernel.org/r/20210406111622.1874957-11-mkl@pengutronix.de
Acked-by: Stephane Grosjean <s.grosjean@peak-system.com>
Tested-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>