docs: usb: convert documents to ReST

Convert USB documents to ReST, in order to prepare for adding it
to the kernel API book, as most of the stuff there are driver or
subsystem-related.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>