can: flexcan: use correct clock as base for bit rate calculation

The flexcan IP core uses the peripheral clock ("per") as basic clock for the
bit timing calculation. However the driver uses the the wrong clock ("ipg").
This leads to wrong bit rates if the rates on both clock are different.

This patch fixes the problem by using the correct clock for the bit rate
calculation.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>