staging:iio:magnetometer:ak8975 use platform_data to pass the gpio number.

Tegra doesn't have irq_to_gpio() any more, and ak8975 is included in
tegra_defconfig. This causes a build failure. Instead, pass the GPIO name
through platform data.

[swarren: Rewrote commit description when I squashed this with my patch
to remove the irq_to_gpio() call]
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/magnetometer/ak8975.c

@@ -477,7 +477,10 @@ static int ak8975_probe(struct i2c_client *client,
 	int err;
 
 	/* Grab and set up the supplied GPIO. */
-	eoc_gpio = irq_to_gpio(client->irq);
+	if (client->dev.platform_data == NULL)
+		eoc_gpio = -1;
+	else
+		eoc_gpio = *(int *)(client->dev.platform_data);
 
 	/* We may not have a GPIO based IRQ to scan, that is fine, we will
 	   poll if so */