mtd: st_spi_fsm: Write to Flash via the FSM FIFO

When we write data to the FIFO the FSM Controller subsequently writes
that data out to the Serial Flash chip.

Acked-by Angus Clark <angus.clark@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>