can: flexcan: flexcan_irq_state(): only read timestamp if needed

The function flexcan_irq_state() checks the controller for CAN state
changes and pushes a skb with the new state and a timestamp into the
rx-offload framework.

This patch optimizes the function by only reading the timestamp, if a
state change is detected.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>