cmd_sf: let "sf update" erase last sector as a whole

make "sf update" work with unaligned `len' parameter, by deleting the
whole last sector before writing, so to allow for:

 sf update ${load_addr_r} 0 ${filesize}

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Cc: Valentin Longchamp <valentin.longchamp@keymile.com>
Cc: Holger Brunck <holger.brunck@keymile.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

common/cmd_sf.c

@@ -160,7 +160,8 @@ static const char *spi_flash_update_block(struct spi_flash *flash, u32 offset,
 		*skipped += len;
 		return NULL;
 	}
-	if (spi_flash_erase(flash, offset, len))
+	/* Erase the entire sector */
+	if (spi_flash_erase(flash, offset, flash->sector_size))
 		return "erase";
 	if (spi_flash_write(flash, offset, len, buf))
 		return "write";