docs-rst: usb: update old usbfs-related documentation

There's no usbfs anymore. The old features are now either
exported to /dev/bus/usb or via debugfs.

Update documentation accordingly, pointing to the new
places where the character devices and usb/devices are
now placed.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/networking/cdc_mbim.txt

@@ -332,7 +332,7 @@ References
 [5] "MBIM (Mobile Broadband Interface Model) Registry"
        - http://compliance.usb.org/mbim/
 
-[6] "/proc/bus/usb filesystem output"
+[6] "/dev/bus/usb filesystem output"
        - Documentation/usb/proc_usb_info.txt
 
 [7] "/sys/bus/usb/devices/.../descriptors"