usb: gadget: add RNDIS configfs options for class/subclass/protocol

This adds 3 new options to the RNDIS gadget function configs. It allows
overriding the default USB interface class/subclass/protocol.

The motivation for this is that if you set the values to "ef" (Misc),
"04" (RNDIS), "01" (Ethernet) respectively, then the device will be
recognized by the rndiscmp.inf file in Windows Vista and newer and will
cause Windows to load the correct RNDIS driver without the need for a
custom (signed) .inf file.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/u_ether_configfs.h

@@ -153,4 +153,39 @@ out:									\
 									\
 	CONFIGFS_ATTR_RO(_f_##_opts_, ifname)
 
+#define USB_ETHER_CONFIGFS_ITEM_ATTR_U8_RW(_f_, _n_)			\
+	static ssize_t _f_##_opts_##_n_##_show(struct config_item *item,\
+					       char *page)		\
+	{								\
+		struct f_##_f_##_opts *opts = to_f_##_f_##_opts(item);	\
+		int ret;						\
+									\
+		mutex_lock(&opts->lock);				\
+		ret = sprintf(page, "%02x\n", opts->_n_);		\
+		mutex_unlock(&opts->lock);				\
+									\
+		return ret;						\
+	}								\
+									\
+	static ssize_t _f_##_opts_##_n_##_store(struct config_item *item,\
+						const char *page,	\
+						size_t len)		\
+	{								\
+		struct f_##_f_##_opts *opts = to_f_##_f_##_opts(item);	\
+		int ret;						\
+		u8 val;							\
+									\
+		mutex_lock(&opts->lock);				\
+		ret = sscanf(page, "%02hhx", &val);			\
+		if (ret > 0) {						\
+			opts->_n_ = val;				\
+			ret = len;					\
+		}							\
+		mutex_unlock(&opts->lock);				\
+									\
+		return ret;						\
+	}								\
+									\
+	CONFIGFS_ATTR(_f_##_opts_, _n_)
+
 #endif /* __U_ETHER_CONFIGFS_H */