can: flexcan: handle state passive -> warning transition

Once the CAN-bus is open and a packet is sent, the controller switches
into the PASSIVE state. Once the BUS is closed again it goes the back
err-warning. The TX error counter goes 0 -> 0x80 -> 0x7f.
This patch makes sure that the user learns about this state chang
(CAN_STATE_ERROR_WARNING => CAN_STATE_ERROR_PASSIVE)

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Matthias Klein <matthias.klein@optimeas.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>