usb: gadget: net2280: Fix overrun of OUT messages

The OUT endpoint normally blocks (NAK) subsequent packets when a
short packet was received and returns an incomplete queue entry to
the gadget driver. Thereby the gadget driver can detect a short packet
when reading queue entries with a length that is not equal to a
multiple of packet size.

The start_queue() function enables receiving OUT packets regardless of
the content of the OUT FIFO. This results in a race: With the current
code, it's possible that the "!ep->is_in && (readl(&ep->regs->ep_stat)
& BIT(NAK_OUT_PACKETS))" test in start_dma() will fail, then a short
packet will be received, and then start_queue() will call
stop_out_naking(). That's what we don't want (OUT naking gets turned
off while there is data in the FIFO) because then the next driver
request might receive a mixture of old and new packets.

With the patch, this race can't occur because the FIFO's state is
tested after we know that OUT naking is already turned on, and OUT
naking is stopped only when both of the conditions are met.  This
ensures that all received data is delivered to the gadget driver,
which can detect a short packet now before new packets are appended
to the last short packet.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>