mtd: st_spi_fsm: kill duplicate CMD definitions

These are also in serial_flash_cmds.h. (FWIW, I didn't know the C
preprocessor allowed redefinitions without warning like this.)

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