MLK-25465-2 tty: serial: imx: keep dma request not enable before dma transfer setup

MLK-25465-2 tty: serial: imx: keep dma request not enable 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 (event enabled
in imx_uart_dma_init() before where's ahead of dma request enable):

"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 cfc4e2c81de12a55414240e0da90a709ac929c3f)