USB: musb: NAK timeout scheme on bulk RX endpoint

Fixes endpoint starvation issue when more than one bulk QH is
multiplexed on the reserved bulk RX endpoint, which is normal
for cases like serial and ethernet adapters.

This patch sets the NAK timeout interval for such QHs, and when
a timeout triggers the next QH will be scheduled.  (This resembles
the bulk scheduling done in hardware by EHCI, OHCI, and UHCI.)

This scheme doesn't work for devices which are connected to a
high to full speed tree (transaction translator) as there is
no NAK timeout interrupt from the musb controller from such
devices.

Tested with PIO, Inventra DMA, CPPI DMA.

[ dbrownell@users.sourceforge.net:  fold in start_urb() update;
  clarify only for bulk RX; don't accidentally clear WZC bits ]

Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Cc: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>