usb: gadget: udc: add a 'function' sysfs file

This file will print out the name of the currently running USB Gadget
Driver. It can be read even when there are no functions loaded.

Suggested-by: Alan Stern <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

Documentation/ABI/stable/sysfs-class-udc

···
 		'configured', and 'suspended'; however not all USB Device
 		Controllers support reporting all states.
 Users:
+
+What:		/sys/class/udc/<udc>/function
+Date:		June 2017
+KernelVersion:	4.13
+Contact:	Felipe Balbi <[e-mail]>
+Description:
+		Prints out name of currently running USB Gadget Driver.
+Users:

drivers/usb/gadget/udc/core.c

···
 }
 static DEVICE_ATTR_RO(state);
 
+static ssize_t function_show(struct device *dev, struct device_attribute *attr,
+			     char *buf)
+{
+	struct usb_udc		*udc = container_of(dev, struct usb_udc, dev);
+	struct usb_gadget_driver *drv = udc->driver;
+
+	if (!drv || !drv->function)
+		return 0;
+	return scnprintf(buf, PAGE_SIZE, "%s\n", drv->function);
+}
+static DEVICE_ATTR_RO(function);
+
 #define USB_UDC_SPEED_ATTR(name, param)					\
 ssize_t name##_show(struct device *dev,					\
 		struct device_attribute *attr, char *buf)		\
···
 	&dev_attr_srp.attr,
 	&dev_attr_soft_connect.attr,
 	&dev_attr_state.attr,
+	&dev_attr_function.attr,
 	&dev_attr_current_speed.attr,
 	&dev_attr_maximum_speed.attr,