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

drivers/net/can/at91_can.c

@@ -576,6 +576,22 @@ static inline void at91_activate_rx_mb(const struct at91_priv *priv,
 	at91_write(priv, AT91_TCR, mask);
 }
 
+static inline u32 at91_get_timestamp(const struct at91_priv *priv)
+{
+	return at91_read(priv, AT91_TIM);
+}
+
+static inline struct sk_buff *
+at91_alloc_can_err_skb(struct net_device *dev,
+		       struct can_frame **cf, u32 *timestamp)
+{
+	const struct at91_priv *priv = netdev_priv(dev);
+
+	*timestamp = at91_get_timestamp(priv);
+
+	return alloc_can_err_skb(dev, cf);
+}
+
 /**
  * at91_rx_overflow_err - send error frame due to rx overflow
  * @dev: net device