serial: core: Add UPIO_UNKNOWN constant for unknown port type

serial: core: Add UPIO_UNKNOWN constant for unknown port type

In some APIs we would like to assign the special value to iotype
and compare against it in another places. Introduce UPIO_UNKNOWN
for this purpose.

Note, we can't use 0, because it's a valid value for IO port access.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240304123035.758700-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>