dw_dmac: don't wait for FIFO_EMPTY endlessly in dwc_chan_pause

When we pause the channel after transfer is completed we might stuck in the
dwc_chan_pause() because the FIFO_EMPTY flag will never be asserted. To avoid
the endless loop we introduce a timeout here (*). The proper solution is to
somehow get the residue in FIFO and avoid busyloop when transfer is done, but
this task is not simple and fast.

Unfortunately we can't use cpu_relax() in conjunction with jiffies checker, due
to we have interrupts disabled by spin_lock_irqsave() and there is a big chance
that no interrupts will come to update the jiffies..

(*) The worst case is
	AHB write * FIFO size / hclk = 5.12 us,
    where
	AHB write = 2 cycles,
	hclk = 100 MHz,
	burst size = 1 byte,
	FIFO size = 256 bytes.
    The proposed 40us timeout might be considered as a big one, though we enter
    to that state only when we have the transfer already completed.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>