tty/serial: Relax the device_type restriction from of_serial

There is no need to test for a device_type property in ns8250
compatible serial ports.  device_type is an OpenFirmware property that
is not required when using the flattened tree representation.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>