usb: ci: Fix gadget reinit

The ChipIdea device controller wasn't properly cleaned up when disabled.
So enabling it again left it in a broken state. The problem occurred for
example when the host unbinds the driver and binds it again.

During the first setup, when the out request is queued, the endpoint is
primed (`epprime`). If the endpoint is then disabled, it stayed primed
with the initial buffer. So after the endpoint is re-enabled, the device
controller and device driver were out of sync: the new out request was
in the driver queue head, yet not submitted, but the "complete" function
was still called, since the endpoint was primed with the old buffer.

With the fastboot function this error led to the (rather confusing)
error message "buffer overflow".

Fixed by clearing the primed buffers with the `epflush` (`ENDPTFLUSH`)
register.

Signed-off-by: Simon Holesch <simon@holesch.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20231120002024.32865-1-simon@holesch.de
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

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