serial: omap: simplify IRQ handling

quite a few changes here, though they are
pretty obvious. In summary we're making sure
to detect which interrupt type we need to
handle before calling the underlying interrupt
handling procedure.

Tested-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>