usb: gadget: f_fs: rename descriptor parsing functions

ffs_do_desc() handles one descriptor, while ffs_do_descs() handles a number
of descriptors. The tho names are so similar that it causes confusion.
Rename to reflect their purpose better.

Signed-off-by: Andrzej Pietrasiewicz <[e-mail]>
Acked-by: Michal Nazarewicz <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/f_fs.c

···
 				   struct usb_descriptor_header *desc,
 				   void *priv);
 
-static int __must_check ffs_do_desc(char *data, unsigned len,
-				    ffs_entity_callback entity, void *priv)
+static int __must_check ffs_do_single_desc(char *data, unsigned len,
+					   ffs_entity_callback entity,
+					   void *priv)
 {
 	struct usb_descriptor_header *_ds = (void *)data;
 	u8 length;
···
 		if (!data)
 			return _len - len;
 
-		ret = ffs_do_desc(data, len, entity, priv);
+		ret = ffs_do_single_desc(data, len, entity, priv);
 		if (unlikely(ret < 0)) {
 			pr_debug("%s returns %d\n", __func__, ret);
 			return ret;