usb: gadget: fastboot: Dequeue the previous IN request for the current request

Recent versions of the fastboot tool will query the partition type before doing
an operation on a partition (such as erase, flash, etc). It will then submit
the operation as soon as the response for the partition type is received.

Usually, the MUSB controller will see that the partition type request return
status was read by the host at the very same time as the actual operation
request is submitted by the host. However, the operation will be read first
(int_rx is handled first in musb_interrupt) and after it is completed, the
fastboot USB gadget driver will send another return status. Hence, this happens
before the musb gadget framework has had a chance to handle the previous
acknowledgement that the host read the return status and dequeue the request.

The host will then usually empty the FIFO by the time musb_interrupt gets around
handling the return status acknowledgement (for the previous request, this is
still on the same musb_interrupt call), so no other interrupt is generated and
the most recent return status acknowledgement remains unaccounted for.

It will then be used as a response for the next command, and the proper response
for it will be delayed to the next command, and so on.

Dequeuing the previous IN request in the fastboot code ensures that no previous
return status remains. It is acceptable to do it since there is no callback to
it anyways.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>

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