flexcan: Prefer device tree clock frequency if available.

- reg : Offset and length of the register set for this device

- interrupts : Interrupt tuple for this device

- clock-frequency : The oscillator frequency driving the flexcan device

Example:

		reg = <0x1c000 0x1000>;

		interrupts = <48 0x2>;

		interrupt-parent = <&mpic>;

		clock-frequency = <200000000>; // filled in by bootloader

	};

#include <linux/kernel.h>

#include <linux/list.h>

#include <linux/module.h>

#include <linux/of.h>

#include <linux/platform_device.h>

#define DRV_NAME			"flexcan"

	struct net_device *dev;

	struct flexcan_priv *priv;

	struct resource *mem;

	struct clk *clk;

	struct clk *clk = NULL;

	void __iomem *base;

	resource_size_t mem_size;

	int err, irq;

	u32 clock_freq = 0;

	if (pdev->dev.of_node) {

		const u32 *clock_freq_p;

		clock_freq_p = of_get_property(pdev->dev.of_node,

						"clock-frequency", NULL);

		if (clock_freq_p)

			clock_freq = *clock_freq_p;

	}

	if (!clock_freq) {

		clk = clk_get(&pdev->dev, NULL);

		if (IS_ERR(clk)) {

			dev_err(&pdev->dev, "no clock defined\n");

			err = PTR_ERR(clk);

			goto failed_clock;

		}

		clock_freq = clk_get_rate(clk);

	}

	mem = platform_get_resource(pdev, IORESOURCE_MEM, 0);

	irq = platform_get_irq(pdev, 0);

	dev->flags |= IFF_ECHO; /* we support local echo in hardware */

	priv = netdev_priv(dev);

	priv->can.clock.freq = clk_get_rate(clk);

	priv->can.clock.freq = clock_freq;

	priv->can.bittiming_const = &flexcan_bittiming_const;

	priv->can.do_set_mode = flexcan_set_mode;

	priv->can.do_get_berr_counter = flexcan_get_berr_counter;

 failed_map:

	release_mem_region(mem->start, mem_size);

 failed_get:

	if (clk)

		clk_put(clk);

 failed_clock:

	return err;

	mem = platform_get_resource(pdev, IORESOURCE_MEM, 0);

	release_mem_region(mem->start, resource_size(mem));

	if (priv->clk)

		clk_put(priv->clk);

	free_candev(dev);