mmc: pxamci: fix potential oops

As reported by Dan in his report in [1], there is a potential NULL
pointer derefence if these conditions are met :
 - there is no platform_data provided, ie. host->pdata = NULL

Fix this by only using the platform data ro_invert when a gpio for
read-only is provided by the platform data.

This doesn't appear yet as every pxa board provides a platform_data, and
calls pxa_set_mci_info() with a non NULL pointer.

[1] [bug report] mmc: pxamci: fix card detect with slot-gpio API.
The commit fd546ee6 ("mmc: pxamci: fix card detect with slot-gpio
API") from Sep 26, 2015, leads to the following static checker warning:

	drivers/mmc/host/pxamci.c:809 pxamci_probe()
	warn: variable dereferenced before check 'host->pdata' (see line 798)

Fixes: fd546ee6 ("mmc: pxamci: fix card detect with slot-gpio API")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/pxamci.c

@@ -789,15 +789,17 @@ static int pxamci_probe(struct platform_device *pdev)
 		gpio_direction_output(gpio_power,
 				      host->pdata->gpio_power_invert);
 	}
-	if (gpio_is_valid(gpio_ro))
+	if (gpio_is_valid(gpio_ro)) {
 		ret = mmc_gpio_request_ro(mmc, gpio_ro);
 		if (ret) {
-		dev_err(&pdev->dev, "Failed requesting gpio_ro %d\n", gpio_ro);
+			dev_err(&pdev->dev, "Failed requesting gpio_ro %d\n",
+				gpio_ro);
 			goto out;
 		} else {
 			mmc->caps2 |= host->pdata->gpio_card_ro_invert ?
 				0 : MMC_CAP2_RO_ACTIVE_HIGH;
 		}
+	}
 
 	if (gpio_is_valid(gpio_cd))
 		ret = mmc_gpio_request_cd(mmc, gpio_cd, 0);