This commit adds dedicated stall functions for usbdev peripherals. Two
functions are added. The first function (usbdev_ep_stall) to enable and
disable the stall condition on generic endpoints. The second function is
a dedicated function to set the stall condition on endpoint zero in both
directions. This status can only be set and should automatically be
cleared by the usbdev implementation (or hardware) after a new setup
request is received from the host.
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