ATmega samples MOSI and shifts out MISO data on rising edge of SCK. Existing
ftdi_syncbb implementation samples MISO data shortly after rising edge of SCK.
This is incorrect, but might work for slow FTDI chip or ATmega with high speed
This patch implements correct MISO sampling, i.e. on falling edge of SCK.
Tested with ATmega88 (1MHz, internal RC) and FT232H.