usb: gadget: f_fs: Document eventfd effect on descriptor format.

When FUNCTIONFS_EVENTFD flag is set, __ffs_data_got_descs reads a 32bits,
little-endian value right after the fixed structure header, and passes it
to eventfd_ctx_fdget. Document this.

Also, rephrase a comment to be affirmative about the role of string
descriptor at index 0. Ref: USB 2.0 spec paragraph "9.6.7 String", and
also checked to still be current in USB 3.0 spec paragraph "9.6.9 String".

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_fs.c

@@ -2091,8 +2091,8 @@ static int __ffs_data_do_entity(enum ffs_entity_type type,
 
 	case FFS_STRING:
 		/*
-		 * Strings are indexed from 1 (0 is magic ;) reserved
-		 * for languages list or some such)
+		 * Strings are indexed from 1 (0 is reserved
+		 * for languages list)
 		 */
 		if (*valuep > helper->ffs->strings_count)
 			helper->ffs->strings_count = *valuep;

include/uapi/linux/usb/functionfs.h

@@ -93,6 +93,7 @@ struct usb_ext_prop_desc {
  * |   0 | magic     | LE32         | FUNCTIONFS_DESCRIPTORS_MAGIC_V2      |
  * |   4 | length    | LE32         | length of the whole data chunk       |
  * |   8 | flags     | LE32         | combination of functionfs_flags      |
+ * |     | eventfd   | LE32         | eventfd file descriptor              |
  * |     | fs_count  | LE32         | number of full-speed descriptors     |
  * |     | hs_count  | LE32         | number of high-speed descriptors     |
  * |     | ss_count  | LE32         | number of super-speed descriptors    |