sf: localize erase funcs

No need for these to be exported as they are only accessed indirectly
via function pointers.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/drivers/mtd/spi/sst.c b/drivers/mtd/spi/sst.c
index 38983bd..0733deb 100644
--- a/drivers/mtd/spi/sst.c
+++ b/drivers/mtd/spi/sst.c
@@ -199,7 +199,7 @@
 	return ret;
 }
 
-int sst_erase(struct spi_flash *flash, u32 offset, size_t len)
+static int sst_erase(struct spi_flash *flash, u32 offset, size_t len)
 {
 	return spi_flash_cmd_erase(flash, CMD_SST_SE, offset, len);
 }