greybus: Fix build errors on older kernels.

Thanks to Marti for pointing out the code didn't build properly on 3.10.
Added kernel_ver.h to handle any api mis-matches between the code and
older kernel versions.

drivers/staging/greybus/gpio-gb.c

@@ -10,7 +10,7 @@
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/gpio.h>
-#include <linux/gpio/driver.h>
+#include <linux/gpio.h>
 #include "greybus.h"
 
 struct gb_gpio_device {
@@ -90,10 +90,11 @@ int gb_gpio_probe(struct greybus_device *gdev,
 void gb_gpio_disconnect(struct greybus_device *gdev)
 {
 	struct gb_gpio_device *gb_gpio_dev;
+	int retval;
 
 	gb_gpio_dev = gdev->gb_gpio_dev;
 
-	gpiochip_remove(&gb_gpio_dev->chip);
+	retval = gpiochip_remove(&gb_gpio_dev->chip);
 	kfree(gb_gpio_dev);
 }
 

drivers/staging/greybus/kernel_ver.h

+/*
+ * Greybus kernel "version" glue logic.
+ *
+ * Copyright 2014 Google Inc.
+ *
+ * Released under the GPLv2 only.
+ *
+ * Backports of newer kernel apis to allow the code to build properly on older
+ * kernel versions.  Remove this file when merging to upstream, it should not be
+ * needed at all
+ */
+
+#ifndef __GREYBUS_KERNEL_VER_H
+#define __GREYBUS_KERNEL_VER_H
+
+#ifndef DEVICE_ATTR_RO
+#define DEVICE_ATTR_RO(_name) \
+	struct device_attribute dev_attr_##_name = __ATTR_RO(_name)
+#endif
+
+
+#endif	/* __GREYBUS_KERNEL_VER_H */

drivers/staging/greybus/sysfs.c

@@ -18,6 +18,8 @@
 
 #include "greybus.h"
 
+#include "kernel_ver.h"
+
 /* Function fields */
 #define greybus_function_attr(field)					\
 static ssize_t function_##field##_show(struct device *dev,		\