channeld: use u64 fo `htlc_minimum_msat`

As per spec.

channeld/channeld.c

@@ -2339,7 +2339,7 @@ static void handle_offer_htlc(struct peer *peer, const u8 *inmsg)
 		goto failed;
 	case CHANNEL_ERR_HTLC_BELOW_MINIMUM:
 		failcode = WIRE_AMOUNT_BELOW_MINIMUM;
-		failmsg = tal_fmt(inmsg, "HTLC too small (%u minimum)",
+		failmsg = tal_fmt(inmsg, "HTLC too small (%"PRIu64" minimum)",
 				  htlc_minimum_msat(peer->channel, REMOTE));
 		goto failed;
 	case CHANNEL_ERR_TOO_MANY_HTLCS:

common/initial_channel.h

@@ -109,7 +109,7 @@ static inline u64 channel_reserve_msat(const struct channel *channel,
  *
  * `htlc_minimum_msat` indicates the smallest value HTLC this node will accept.
  */
-static inline u32 htlc_minimum_msat(const struct channel *channel,
+static inline u64 htlc_minimum_msat(const struct channel *channel,
 				    enum side recipient)
 {
 	return channel->config[recipient]->htlc_minimum_msat;