mmc: sunxi: Use devm_reset_control_get_optional() for reset control

commit 9e71c589 upstream.

The reset control for the sunxi mmc controller is optional. Some
newer platforms (sun6i, sun8i, sun9i) have it, while older ones
(sun4i, sun5i, sun7i) don't.

Use the properly stubbed _optional version so the driver does not
fail to compile when RESET_CONTROLLER=n.

This patch also adds a check for deferred probing on the reset
control.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: David Lanzendörfer <david.lanzendoerfer@o2s.ch>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/mmc/host/sunxi-mmc.c

@@ -909,7 +909,9 @@ static int sunxi_mmc_resource_request(struct sunxi_mmc_host *host,
 		return PTR_ERR(host->clk_mmc);
 	}
 
-	host->reset = devm_reset_control_get(&pdev->dev, "ahb");
+	host->reset = devm_reset_control_get_optional(&pdev->dev, "ahb");
+	if (PTR_ERR(host->reset) == -EPROBE_DEFER)
+		return PTR_ERR(host->reset);
 
 	ret = clk_prepare_enable(host->clk_ahb);
 	if (ret) {