can: xilinx_can: only report warning and passive states on state changes

The xilinx_can driver currently increments error-warning and
error-passive statistics on every error interrupt regardless of whether
the interface was already in the same state. Similarly, the error frame
sent on error interrupts is always sent with
CAN_ERR_CRTL_(RX|TX)_(PASSIVE|WARNING) bit set.

To make the error-warning and error-passive statistics more useful, add
a check to only set the error state when the state has actually been
changed.

Tested with the integrated CAN on Zynq-7000 SoC.

Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>