mtd: spi-nor: spansion: write 2 bytes when disabling Octal DTR mode

The Octal DTR configuration is stored in the CFR5V register. This
register is 1 byte wide. But 1 byte long transactions are not allowed in
8D-8D-8D mode. Since the next byte address does not contain any
register, it is safe to write any value to it. Write a 0 to it.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20210531181757.19458-3-p.yadav@ti.com