spi: spi-mtk-nor: make mtk_nor_exec_op() statuc

Fix the following sparse warning:

drivers/spi/spi-mtk-nor.c:394:5: warning: symbol 'mtk_nor_exec_op' was
not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200409085009.44971-1-yanaijie@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>