mmc: sdhci-brcmstb: Fix driver to defer on clk_get defer

The new SCMI clock protocol driver does not get probed that early in
boot. Brcmstb drivers typically have the following code when getting
a clock:

        priv->clk = devm_clk_get(&pdev->dev, NULL);
        if (IS_ERR(priv->clk)) {
                dev_err(&pdev->dev, "Clock not found in Device Tree\n");
                priv->clk = NULL;
        }

This commit changes the driver to do what is below.

        priv->clk = devm_clk_get(&pdev->dev, NULL);
        if (IS_ERR(priv->clk)) {
                if (PTR_ERR(priv->clk) == -EPROBE_DEFER)
                        return -EPROBE_DEFER;
                dev_err(&pdev->dev, "Clock not found in Device Tree\n");
                priv->clk = NULL;
        }

Signed-off-by: Al Cooper <alcooperx@gmail.com>
Link: https://lore.kernel.org/r/20200113210706.11972-4-alcooperx@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>