USB: serial: pl2303: add support for PL256X multi-port devices

Prolific PL256X devices are multi-port USB-to-UART controllers,
including the PL2533, PL2543 and PL2565 variants.

These devices use vendor requests that differ from those used by the
existing TYPE_HX and TYPE_HXN devices. They also require a separate UART
reset request and use a port-specific register for configuring flow
control.

Add a new TYPE_MP device type and select the appropriate vendor requests,
reset operation and flow-control register for PL256X devices. Store the
USB interface number so that requests can be directed to the corresponding
UART port.

Detect the supported PL256X variants using bcdDevice before issuing any
legacy vendor requests, as PL256X devices do not accept those requests.

PL256X devices support baud rates up to 24 Mbps and do not use divisor
encoding.

Signed-off-by: Charles Yeh <charlesyeh522@gmail.com>
Link: https://lore.kernel.org/all/CAAZvQQ6O4p35Xs2hVYaoJxD4D7U0YonsdweuPh6W8RQVhvoUNw@mail.gmail.com/
Signed-off-by: Johan Hovold <johan@kernel.org>

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