Documentation: driver-api: usb: Limit toctree depth

toctree index in USB driver api docs currently spoils the entire docs
headings due to lack of :maxdepth: option. Add the option to limit
toctree depth to 1, mirroring usb subsystem docs in
Documentation/usb/index.rst.

Signed-off-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/driver-api/usb/index.rst

···
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 .. toctree::
+   :maxdepth: 1
 
    usb
    gadget