serial: 8250_omap: Drop check for of_node

8250_omap is DT only driver so dev->of_node always exists. Drop check
for existence of valid dev->of_node to simplify omap8250_probe().

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>