This is a preparation patch to convert the driver to the rx-offload
helper. In rx-offload RX, TX-done and CAN error handling are done in
the IRQ handler, SKB are pushed to the network stack in the NAPI poll
function.

Move the CAN frame error handling from the NAPI function at91_poll()
to the IRQ handler at91_poll(). To reflect this change, rename
at91_poll_err() to at91_irq_err_frame().

Link: https://lore.kernel.org/all/20231005-at91_can-rx_offload-v2-19-9987d53600e0@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>