mtd: st_spi_fsm: replace FLACH_CMD_* with SPINOR_OP_*

Begin to unify the differences between serial_flash_cmds.h and
spi-nor.h.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Marek Vasut <marex@denx.de>