usb: Restore the reference to ch9.h

Keep the textual reference to ch9.h as it was prior to commit
caa93d9b ("usb: Fix up movement of USB core kerneldoc location").

As linux/usb/ch9.h does not contain comments anymore, explain
that drivers/usb/common/common.c includes such header and provides
declarations of a few utilities routines for manipulating the data types
from ch9.h. Also mention that drivers/usb/common/debug.c contains
some functions for creating debug output.

Fixes: caa93d9b

 ("usb: Fix up movement of USB core kerneldoc location")
Reported-by: Alan Stern <stern@rowland.harvard.edu>
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210425153253.2542816-1-festevam@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>