serial: 8250: extract serial8250_iir_txen_test()

serial: 8250: extract serial8250_iir_txen_test()

After commit 795158691cc0 ("serial: 8250: extract
serial8250_initialize()"), split serial8250_initialize() even more --
the TX enable test part of this code can be separated into
serial8250_iir_txen_test().

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20250624080641.509959-3-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>