mtd: spi-nor: spansion: preserve CFR2V[7] when writing MEMLAT

CFR2V[7] is assigned to Flash's address mode (3- or 4-ybte) and must not
be changed when writing MEMLAT (CFR2V[3:0]). CFR2V shall be used in a read,
update, write back fashion.

Fixes: c3266af1

 ("mtd: spi-nor: spansion: add support for Cypress Semper flash")
Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230726075257.12985-3-tudor.ambarus@linaro.org
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>