MLK-25465-4 spi: imx: keep dma request disabled before dma transfer setup

MLK-25465-4 spi: imx: keep dma request disabled before dma transfer setup

Since sdma hardware configure postpone to transfer phase, have to disable
dma request on uart driver before dma transfer setup because there is
a hardware limitation on sdma event enable(ENBLn) as below:

"It is thus essential for the Arm platform to program them before any DMA
 request is triggered to the SDMA, otherwise an unpredictable combination
 of channels may be started."

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Reviewed-by: Shengjiu Wang <shengjiu.wang@nxp.com>
(cherry picked from commit 835afd7712926454edce6f9915a5f7f3a9cf782e)