MLK-9776: dma: imx-sdma: make sure BUG_ON workaround do not work in loop mode

  The earlier patch for "ENGR00313512 dma: imx-sdma: A bungle of work around for
BUG ON issue" is better not applied for loop mode, since there is no any cookie
touch in loop mode which means never trigger the BUG_ON workaround. Otherwise,
if upper driver such as UART disable dma channel but the device still has sent
the data into the RXFIFO, bd buffer will be consumed quickly and then stop.That
will trigger "RX FIFO overrun", like below log:

imx-uart 21ec000.serial: Rx FIFO overrun
imx-uart 21ec000.serial: Rx FIFO overrun
imx-uart 21ec000.serial: Rx FIFO overrun
imx-uart 21ec000.serial: Rx FIFO overrun
imx-uart 21ec000.serial: Rx FIFO overrun
imx-uart 21ec000.serial: Rx FIFO overrun
imx-uart 21ec000.serial: Rx FIFO overrun
imx-uart 21ec000.serial: Rx FIFO overrun

Signed-off-by: Robin Gong <b38343@freescale.com>
(cherry picked from commit ac31647b11c3e441901fa116d74ebef9051e22e5)