can: xilinx_can: update stats.tx_bytes after transmission

The driver updates stats.tx_bytes in start_xmit() even though it could
do so in TX interrupt handler.

Change the code to update tx_bytes in the interrupt handler, using the
return value of can_get_echo_skb().

Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>