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>

include/linux/serial_core.h

@@ -470,6 +470,7 @@ struct uart_port {
 
 	unsigned char		iotype;			/* io access style */
 
+#define UPIO_UNKNOWN		((unsigned char)~0U)	/* UCHAR_MAX */
 #define UPIO_PORT		(SERIAL_IO_PORT)	/* 8b I/O port access */
 #define UPIO_HUB6		(SERIAL_IO_HUB6)	/* Hub6 ISA card */
 #define UPIO_MEM		(SERIAL_IO_MEM)		/* driver-specific */