USB: serial: ftdi_sio: add support for FT232H CBUS gpios

Enable support for cbus gpios on FT232H. The cbus configuration is
stored in two words in the EEPROM at byte-offset 0x1a with the mux
config for ACBUS5, ACBUS6, ACBUS8 and ACBUS9 (only pins that can be
configured as I/O mode).

Tested using FT232H by configuring one ACBUS pin at a time.

Reviewed-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Matthew Michilot <matthew.michilot@gmail.com>
[ johan: fix copy-paste error in commit message ]
Signed-off-by: Johan Hovold <johan@kernel.org>