USB: serial: f81534: add output pin control

The F81532/534 had 3 output pin (M0/SD, M1, M2) with open-drain mode to
control transceiver. We'll read it from internal Flash with address
0x2f05~0x2f08 for 4 ports. The value is range from 0 to 7. The M0/SD is
MSB of this value. For a examples, If read value is 6, we'll write M0/SD,
M1, M2 as 1, 1, 0.

Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>