spi: spi-mtk-nor: Unify write buffer on/off

The logical structures of mtk_nor_write_buffer_enable and
mtk_nor_write_buffer_disable are very similar, So it is necessary to
combine them into one.

Signed-off-by: bayi cheng <bayi.cheng@mediatek.com>
Link: https://lore.kernel.org/r/20221115124655.10124-1-bayi.cheng@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>