can: flexcan: increase error counters if skb enqueueing via can_rx_offload_queue_sorted() fails

The call to can_rx_offload_queue_sorted() may fail and return an error
(in the current implementation due to resource shortage). The passed skb
is consumed.

This patch adds incrementing of the appropriate error counters to let
the device statistics reflect that there's a problem.

Reported-by: Martin Hundebøll <martin@geanix.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>