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>

drivers/mtd/spi/spi_flash.c

@@ -233,7 +233,7 @@ int spi_flash_cmd_erase(struct spi_flash *flash, u8 erase_cmd,
 			goto out;
 	}
 
-	debug("SF: Successfully erased %lu bytes @ %#x\n",
+	debug("SF: Successfully erased %zu bytes @ %#x\n",
 	      len * erase_size, start);
 
  out: