can: at91_can: at91_poll_err(): fold in at91_poll_err_frame()

This is a preparation patch for the cleanup of at91_poll_err(). Fold
at91_poll_err_frame() into at91_poll_err() so that it can be easier
modified.

Link: https://lore.kernel.org/all/20231005-at91_can-rx_offload-v2-17-9987d53600e0@pengutronix.de


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