mtd/nvs: Fix the issue of compilation failure using the tasking compiler

the tasking compiler reports the error: expression must be constant
in line 510

Signed-off-by: zhaoxingyu1 <zhaoxingyu1@xiaomi.com>

drivers/mtd/mtd_config_nvs.c

@@ -507,7 +507,7 @@ static int nvs_flash_direct_cmp(FAR struct nvs_fs *fs, uint32_t addr1,
                                 uint32_t addr2, size_t len)
 {
   uint8_t buf1[nvs_buffer_size(fs)];
-  uint8_t buf2[sizeof(buf1)];
+  uint8_t buf2[nvs_buffer_size(fs)];
   size_t bytes_to_cmp;
   int rc;