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.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-mtk-nor.c

@@ -391,7 +391,7 @@ static int mtk_nor_pp_unbuffered(struct mtk_nor *sp,
 	return mtk_nor_cmd_exec(sp, MTK_NOR_CMD_WRITE, 6 * BITS_PER_BYTE);
 }
 
-int mtk_nor_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
+static int mtk_nor_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
 {
 	struct mtk_nor *sp = spi_controller_get_devdata(mem->spi->master);
 	int ret;