libmctp: Clarify BTU-related symbols

DSP0236 defines a Baseline Transmission Unit (BTU), which dictates the
minimum supported packet payload size. The packet size is the BTU + the
packet header. The maximum (payload) transport unit size for a given
binding may be larger than the BTU.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I2c4013531efd572d95c5a61e4d59309490505a8f
diff --git a/serial.c b/serial.c
index 808c215..c90de54 100644
--- a/serial.c
+++ b/serial.c
@@ -347,7 +347,7 @@
 	serial->rx_pkt = NULL;
 	serial->binding.name = "serial";
 	serial->binding.version = 1;
-	serial->binding.pkt_size = MCTP_BMTU;
+	serial->binding.pkt_size = MCTP_PACKET_SIZE(MCTP_BTU);
 	serial->binding.pkt_pad = 0;
 
 	serial->binding.start = mctp_serial_core_start;