docs: usb: remove some broken references

It seems that some files were removed from USB documentation.

Update the links accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/00008303fde6b4e06d027d3b76ae7032614a7030.1580193653.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>