can: at91_can: at91_alloc_can_err_skb() introduce new function

This is a preparation patch to convert the driver to make use of the
rx-offload helper. With rx-offload the received CAN frames are sorted
by their timestamp. Regular CAN RX'ed and TX'ed CAN frames are
timestamped by the hardware. Error events are not.

Introduce a new function at91_alloc_can_err_skb() the allocates an
error SKB and reads the current timestamp from the controller.

Link: https://lore.kernel.org/all/20231005-at91_can-rx_offload-v2-26-9987d53600e0@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>