astlpc: Initialise with at least a BTU-sized packet

The astlpc binding's MTU negotiation takes place during processing of
the `Channel Init` command, after which both sides update `pkt_size`
according to the negotiated MTU. Despite this, let's not set a
potentially invalid MTU at binding construction.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Ic6d3dba38102b81ff9b7c09865f359d31ee829f7
diff --git a/astlpc.c b/astlpc.c
index e8b3cd5..f58052a 100644
--- a/astlpc.c
+++ b/astlpc.c
@@ -970,7 +970,8 @@
 	astlpc->requested_mtu = mtu;
 	astlpc->binding.name = "astlpc";
 	astlpc->binding.version = 1;
-	astlpc->binding.pkt_size = MCTP_PACKET_SIZE(mtu);
+	astlpc->binding.pkt_size =
+		MCTP_PACKET_SIZE(mtu > MCTP_BTU ? mtu : MCTP_BTU);
 	astlpc->binding.pkt_pad = 0;
 	astlpc->binding.tx = mctp_binding_astlpc_tx;
 	if (mode == MCTP_BINDING_ASTLPC_MODE_BMC)