tty: omap-serial: Enable IDLE MODE on runtime suspend

Commit f905c6a0fa5961f6101dbfff837c24c1fdbd2592 forbid idle for uart on
runtime resume path if dma is enabled. This patch allow idle on runtime
suspend path.

Change-Id: If28a79695ad29c07f634ed3ef71f9c9842f27188
Signed-off-by: Vitaly Chernooky <vitaly.chernooky@ti.com>