can: flexcan: make use of rx-offload's irq_offload_fifo

This patch converts the flexcan driver to make use of the rx-offload
can_rx_offload_irq_offload_fifo() helper function. The idea is to read
the CAN frames already in the interrupt context, as the depth of the
flexcan HW FIFO is too shallow, resulting in too many missed frames.
During a normal NAPI poll the frames are the pushed into the upper
layers.

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