[netif] use fix type to u8 for addr_gen_mode (#9147)

From the kernel code, addr_gen_mode should be uint8_t.

src/posix/platform/netif.cpp

@@ -1686,7 +1686,7 @@ static void SetAddrGenModeToNone(void)
         char             buf[512];
     } req;
 
-    const enum in6_addr_gen_mode mode = IN6_ADDR_GEN_MODE_NONE;
+    const uint8_t mode = IN6_ADDR_GEN_MODE_NONE;
 
     memset(&req, 0, sizeof(req));