can: rx-offload: can_rx_offload_offload_one(): use ERR_PTR() to propagate error value in case of errors

Before this patch can_rx_offload_offload_one() returns a pointer to a
skb containing the read CAN frame or a NULL pointer.

However the meaning of the NULL pointer is ambiguous, it can either mean
the requested mailbox is empty or there was an error.

This patch fixes this situation by returning:
- pointer to skb on success
- NULL pointer if mailbox is empty
- ERR_PTR() in case of an error

All users of can_rx_offload_offload_one() have been adopted, no
functional change intended.
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>