mtd: correct typo "MTD_DATAFLASH_VERIFY_WRITE"

Fix the misspelling to match the actual config variable defined in
drivers/mtd/devi ces/Kconfig:

config MTD_DATAFLASH_WRITE_VERIFY
        bool "Verify DataFlash page writes"
        depends on MTD_DATAFLASH

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>