Transceiver: Clean up receival of downlink bursts

Use a packed structure to clearly indicate what is contained in the
received buffer.

Change-Id: I4d8c0e3c0c717699889f79e50c778d14b6058f2d
diff --git a/Transceiver52M/proto_trxd.h b/Transceiver52M/proto_trxd.h
index 7eb5dac..d72cfdd 100644
--- a/Transceiver52M/proto_trxd.h
+++ b/Transceiver52M/proto_trxd.h
@@ -66,6 +66,13 @@
 	uint8_t soft_bits[0];
 } __attribute__ ((packed));
 
+/* Downlink burst (BTS->TRX), v0 anf v1 use same format */
+struct trxd_hdr_v01_dl {
+	struct trxd_hdr_common common;
+	uint8_t tx_att; /* Tx Attentuation */
+	uint8_t soft_bits[0];
+} __attribute__ ((packed));
+
 
 #define TRXD_MODULATION_GMSK(ts_set) (0b0000 | (ts_set & 0b0011))
 #define TRXD_MODULATION_8PSK(ts_set) (0b0100 | (ts_set & 0b0001))