serial: 8250: Document serial8250_modem_status() locking

Existing callers of serial8250_modem_status() [1] hold the uart port
lock; document.

[1] In-tree callers of serial8250_modem_status()

drivers/tty/serial/8250/8250_fsl.c
  fsl8250_handle_irq()

drivers/tty/serial/8250/8250_core.c
  serial8250_handle_irq()
  serial8250_console_write()
  serial8250_get_mctrl() *

* Call graphs for callers of serial8250_get_mctrl() from the function
  which acquires the uart port lock

drivers/tty/serial/serial_core.c
  uart_port_startup()
  uart_tiocmget()
  uart_set_termios()
  uart_carrier_raised()
    ops->get_mctrl() ---> serial8250_get_mctrl()

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>