USB: serial: f81232: add control driver for F81534A

The Fintek F81534A series contains 1 HUB, 1 GPIO device and n UARTs. The
UARTs are disabled by default and need to be enabled by the GPIO device
(2c42:16F8).

When F81534A plug to host, we can only see 1 HUB and 1 GPIO device and
we write 0x8fff to GPIO device register F81534A_CTRL_CMD_ENABLE_PORT
(116h) to enable all available serial ports.

Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
[johan: reword commit message and an error message slightly]
Signed-off-by: Johan Hovold <johan@kernel.org>