can: sja1000: fix network statistics update

The member "tx_bytes" of "struct net_device_stats" should be
incremented when the interrupt is done and an "arbitration
lost error" is a TX error and the statistics should be updated
accordingly.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>