connect AR9271(USB wifi) to AM335x, and send a flood ping from Mac OSX,
AR9271 is stopped.

on USB bus, the following occurs.

 - OUT transaction is ACKed (NYET).
 - IN transaction is ACKed (512bytes).
 - PING-NAK transaction is continued for about 2 seconds (AR9271 timeout?).

In current imprementation, IN-transaction is not completed because it
checks the empty of TX-FIFO in cppi41_dma_callback().
As a result, communication to AR9271 stops.

This patch modified to check the empty of TX-FIFO only when OUT-transaction.
Signed-off-by: Takeyoshi Kikuchi <kikuchi@centurysys.co.jp>
Signed-off-by: Felipe Balbi <balbi@ti.com>