sf: fix debug format string warning

On some systems, we get a warning when %lu is used with size_t's, so
use the correct format string.

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/drivers/mtd/spi/spi_flash.c b/drivers/mtd/spi/spi_flash.c
index 730c009..ced4c94 100644
--- a/drivers/mtd/spi/spi_flash.c
+++ b/drivers/mtd/spi/spi_flash.c
@@ -233,7 +233,7 @@
 			goto out;
 	}
 
-	debug("SF: Successfully erased %lu bytes @ %#x\n",
+	debug("SF: Successfully erased %zu bytes @ %#x\n",
 	      len * erase_size, start);
 
  out: