st gets popped from queue head for hw dequeue
       G) requets from queue head wasn't enqueued
               g1)  isr_tr_complete_low() doesn't
               enqueue more requests and it doesn't prime EP,
               rx traffic stalls

Solution:
       a) enque queued requests ASAP, i.e. before calling complete() callback.
       b) don't HW enqueue and prime endpoint with recently added request and
       use the oldest request in the queue.

Fixed issues:
       a) ep_queue() may return an error code despite request was successfully
       added to the queue (if _hardware_enqueue() fails)
       b) Added requests are always processed in LIFO order, even if they are
       added in complete() callback
       c) Finally more than two and more queued requests are processed consistently,
       even if they were added in complete() callback

The fix was successfully tested on MIPS based SoC with 4KEc CPU core and
CI13612 USB core. Board successfully boots with NFS root using g_ether
on ci13xxx udc.

Signed-off-by: Artem Leonenko <tikkeri@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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