mtd: spi-nor: spansion: Enable JFFS2 write buffer for S25FS256T

Infineon(Cypress) SEMPER NOR flash family has on-die ECC and its program
granularity is 16-byte ECC data unit size. JFFS2 supports write buffer
mode for ECC'd NOR flash. Provide a way to clear the MTD_BIT_WRITEABLE
flag in order to enable JFFS2 write buffer mode support. Drop the
comment as the same info is now specified in cypress_nor_ecc_init().

Fixes: 6afcc840 ("mtd: spi-nor: spansion: Add support for Infineon S25FS256T")
Suggested-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/641bfb26c6e059915ae920117b7ec278df1a6f0a.1680760742.git.Takahiro.Kuwano@infineon.com


Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>