serial: 8250_ni: use serial_port_in()/serial_port_out() helpers

serial: 8250_ni: use serial_port_in()/serial_port_out() helpers

There are serial_port_in()/serial_port_out() helpers to be used
instead of direct p->serial_in()/p->serial_out().

Use them in various 8250 drivers.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Chaitanya Vadrevu <chaitanya.vadrevu@emerson.com>
Reviewed-by: Chaitanya Vadrevu <chaitanya.vadrevu@emerson.com>
Link: https://lore.kernel.org/r/20250321182119.454507-6-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>