ft2232_spi: Fix broken GPIOL cs_bits state (#126)

This only sets 3rd CS# bit be asserted during read/write operations.

Tested and confirmed working on 4232H & PicoTap ft2232 programmers
against MX25R6435F & S25FL128S chips.

Signed-off-by: Samir Ibradzic <sibradzic@gmail.com>
Change-Id: Ia0ac14b9a52f251306887500dae3e57d73322157
Reviewed-on: https://review.coreboot.org/c/flashrom/+/38898


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>