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>