dwc_otg: fiq_fsm: pause when cancelling split transactions

Non-periodic splits will DMA to/from the driver-provided transfer_buffer,
which may be freed immediately after the dequeue call returns. Block until
we know the transfer is complete.

A similar delay is needed when cleaning up disconnects, as the FIQ could
have started a periodic transfer in the previous microframe to the one
that triggered a disconnect.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.org>