STM32F2xx: Data from SPI Slave to Master
The master always controls the clock. In the most basic SPI implementation, the master clocks 8 times and the slave outputs a single byte. If the slave has more output, the master must ask for it.

It is possible to have an external interrupt that the slave triggers when it has data for the master, but again the master controls the clock and must clock the data out of the slave. But this only works if the master supports such an interrupt, which the FT232H does not.