Fix GATT MTU truncation in the max_len calculation.

diff --git a/tools/mesh/gatt.c b/tools/mesh/gatt.c
index 0a942d4..c9e789c 100644
--- a/tools/mesh/gatt.c
+++ b/tools/mesh/gatt.c
@@ -189,7 +189,7 @@ static bool sock_write(struct io *io, void *user_data)
 	struct write_data *data = user_data;
 	struct iovec iov[2];
 	uint8_t sar;
-	uint8_t max_len;
+	uint16_t max_len;
 
 	if (data == NULL)
 		return true;