serial: 8250: Decouple core from skip_txen_test module param

Prepare for 8250 split; initialize the port flags based on the
"skip_txen_test" module param to use the existing flag test.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>