USB: serial: ftdi_sio: add support for FT232R CBUS gpios

Enable support for cbus gpios on FT232R. The cbus configuration is
stored in one word in the EEPROM at offset 0x0a (byte-offset 0x14) with
the mux config for CBUS0, CBUS1, CBUS2 and CBUS3 in bits 0..3, 4..7,
8..11 and 12..15, respectively.

Tested using FT232RL by configuring one cbus pin at a time.

Signed-off-by: Johan Hovold <johan@kernel.org>