usb: Fix up movement of USB core kerneldoc location

Commit 855b35ea96c4 ("usb: common: move function's kerneldoc next to its
definition") moved the USB common function documentation out of the
linux/usb/ch9.h header file into drivers/usb/common/common.c and
drivers/usb/common/debug.c, which causes the following 'make htmldocs'
build warning:

include/linux/usb/ch9.h:1: warning: no structured comments found

Fix that up by pointing the documentation at the correct location.

Fixes: 855b35ea96c4 ("usb: common: move function's kerneldoc next to its definition")
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NFabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210424135103.2476670-1-festevam@gmail.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/usb/usb.rst

@@ -109,15 +109,16 @@ well as to make sure they aren't relying on some HCD-specific behavior.
 USB-Standard Types
 ==================
 
-In ``<linux/usb/ch9.h>`` you will find the USB data types defined in
-chapter 9 of the USB specification. These data types are used throughout
-USB, and in APIs including this host side API, gadget APIs, usb character
-devices and debugfs interfaces.
+In ``drivers/usb/common/common.c`` and ``drivers/usb/common/debug.c`` you
+will find the USB data types defined in chapter 9 of the USB specification.
+These data types are used throughout USB, and in APIs including this host
+side API, gadget APIs, usb character devices and debugfs interfaces.
 
-.. kernel-doc:: include/linux/usb/ch9.h
-   :internal:
+.. kernel-doc:: drivers/usb/common/common.c
+   :export:
 
-.. _usb_header:
+.. kernel-doc:: drivers/usb/common/debug.c
+   :export:
 
 Host-Side Data Types and Macros
 ===============================