spi: falcon: mark falcon_sflash_xfer() as static

Fix the following missing-prototypes build warning:

drivers/spi/spi-falcon.c:97:5: error: no previous prototype for 'falcon_sflash_xfer' [-Werror=missing-prototypes]
   97 | int falcon_sflash_xfer(struct spi_device *spi, struct spi_transfer *t,
      |     ^~~~~~~~~~~~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://patch.msgid.link/OSBPR01MB16705BE87E549B6210CD6BCABC72A@OSBPR01MB1670.jpnprd01.prod.outlook.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: DragonBluep

drivers/spi/spi-falcon.c

@@ -94,8 +94,9 @@ struct falcon_sflash {
 	struct spi_controller *host;
 };
 
-int falcon_sflash_xfer(struct spi_device *spi, struct spi_transfer *t,
-		unsigned long flags)
+static int
+falcon_sflash_xfer(struct spi_device *spi, struct spi_transfer *t,
+		   unsigned long flags)
 {
 	struct device *dev = &spi->dev;
 	struct falcon_sflash *priv = spi_controller_get_devdata(spi->controller);