Revert "Revert "omap: serial: disable serial port irq during suspend""

This reverts commit 5b1474a145ed8bde718c0e4a079249e1160a053c.

The regression introduced with this commit will be corrected in
another commit fixing the irq handling on suspend.

Change-Id: Ib828434045459a0593b65c49f9c8484a61ba252c
omap4-common: only disable interrupt when port is suspending

Previously the interrupt got disabled and then the suspend
could be aborted due to data in the rx fifo. When this happened
the interrupt was never enabled again.

Change-Id: I3d145b48b56f5ef69aa062e36e4dff3ed5bdff42