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: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210424135103.2476670-1-festevam@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>