can: flexcan: use be32_to_cpup to handle the value of dt entry

The freescale arm i.MX series platform can support this driver, and
usually the arm cpu works in the little endian mode by default, while
device tree entry value is stored in big endian format, we should use
be32_to_cpup() to handle them, after modification, it can work well
both on the le cpu and be cpu.

Cc: stable <stable@vger.kernel.org> # v3.2+
Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Hui Wang <jason77.wang@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>