can: sun4i: handle overrun in RX FIFO

SUN4Is CAN IP has a 64 byte deep FIFO buffer. If the buffer is not
drained fast enough (overrun) it's getting mangled. Already received
frames are dropped - the data can't be restored.

Signed-off-by: Gerhard Bertelsmann <info@gerhard-bertelsmann.de>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>