serial: add PORT_GENERIC definition

Current pattern in the linux kernel is that every new serial driver adds
one or more new PORT_ definitions because uart_ops::config_port()
callback documentation prescribes setting port->type according to the
type of port found, or to PORT_UNKNOWN if no port was detected.

When the specific type of the port is not important to the userspace
there's no need for a unique PORT_ value, but so far there's no suitable
identifier for that case.

Provide generic port type identifier other than PORT_UNKNOWN for ports
which type is not important to userspace.

Suggested-by: Arnd Bergmann <[e-mail]>
Suggested-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Max Filippov <[e-mail]>
Suggested-by: Jiri Slaby <[e-mail]>
Reviewed-by: Jiri Slaby <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

include/uapi/linux/serial_core.h

···
 /* Sunplus UART */
 #define PORT_SUNPLUS	123
 
+/* Generic type identifier for ports which type is not important to userspace. */
+#define PORT_GENERIC	(-1)
+
 #endif /* _UAPILINUX_SERIAL_CORE_H */