ar7: fix ac49x compilation with 4.9

This will introduce a warning on 3.18, but fixes an error when
compiling for 4.9.

Fixes: cf9e0a59aa ("ar7: add kernel 4.9 support")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>

target/linux/ar7/files/drivers/mtd/ac49xpart.c

@@ -123,7 +123,7 @@ void gen_partname(unsigned int type,
 }
 
 static int create_mtd_partitions(struct mtd_info *master,
-				 struct mtd_partition **pparts,
+				 const struct mtd_partition **pparts,
 				 struct mtd_part_parser_data *data)
 {
 	unsigned int envpartnum = 0, linuxpartnum = 0;