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>