can: mcp25xxfd: Add CAN transmission support

Add un-optimized CAN2.0 and CAN-FD transmission support.

On a Rpi3 we can saturate the CAN bus at 1MHz transmitting CAN2.0
frames with DLC=0 for the number of configured tx-fifos.
Afterwards we need some time to recover the state before we can
fill in the fifos again.

With 7 tx fifos we can: send those 7 frames in 0.33ms and then we
wait for 0.26ms so that is a 56% duty cycle.

With 24 tx fifos this changes to: 1.19ms for 24 frames and then we
wait for 0.52ms so that is a 70% duty cycle.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>