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All of this callbacks which I remove here are not implemented and return
an error code. The gadget code returns an error code if a callback is
missing so there is no need to implement this twice.

Cc: Li Yang <leoli@freescale.com>
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Felipe Balbi <balbi@ti.com>