can: flexcan: use of_property_read_u32 to get DT entry value

of_property_read_u32() can auto handle endian problems, use this
function can make code clean and simple.

No need to check return value here since the following got value
check will handle this.

Cc: linux-can@vger.kernel.org
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: Wolfgang Grandegger <wg@grandegger.com>
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>