serial: doc: Always refer to tty_port->mutex

Stop referring to the mutex member of the tty_port struct as
'port->mutex', as 'port' is ambiguous, and usually refers to the
uart_port struct in this document.  Use 'tty_port->mutex' instead,
following the single existing use.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>