tty: omap-serial: Disabling DMA on suspend path

In case of bug UART can't suspend with TX DMA enabled. So here is
implemented WA: TX DMA is disabled on runtime suspend path and
re-enabled on runtime resume path.

Change-Id: Iabc726ca93a5e354eb53efdacedf3261d4e02763
Signed-off-by: Vitaly Chernooky <vitaly.chernooky@ti.com>