serial: core: add comment about definitely used port types

When port type 18 was removed, it was deduced that the code could go but
its define has to stay because it is used in userspace. Share that
knowledge by adding a comment about it.

Signed-off-by: Wolfram Sang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

include/uapi/linux/serial_core.h

···
  * The type definitions.  These are from Ted Ts'o's serial.h
  * By historical reasons the values from 0 to 13 are defined
  * in the include/uapi/linux/serial.h, do not define them here.
+ * Values 0 to 19 are used by setserial from busybox and must never
+ * be modified.
  */
 #define PORT_NS16550A	14
 #define PORT_XSCALE	15