USB: serial: mos7840: document MCS7810 detection hack

Document the MCS7810 detection hack which relies on having the GPO and
GPI pins connected as recommended by ASIX.

Note that GPO (pin 42) is really RTS of the third port which will be
toggled for the corresponding physical port on two- and four-port
devices.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>