tty: fix typo in ASYNCB_FOURPORT comment

On #define ASYNCB_FOURPORT there's an ortography error on comment:
"Set OU1, OUT2 per AST Fourport settings"

Change it into:
"Set OUT1, OUT2 per AST Fourport settings"

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>