Fix encoding of BSSGP_IE_NUM_OCT_AFF in bssgp_tx_llc_discarded()

This is a 24bit integer value, and thus we need to encode the three
least significant octets of the network-byte-order 32bit value, not the
three most significant octets.
diff --git a/src/gb/gprs_bssgp_bss.c b/src/gb/gprs_bssgp_bss.c
index c058850..9d0325a 100644
--- a/src/gb/gprs_bssgp_bss.c
+++ b/src/gb/gprs_bssgp_bss.c
@@ -230,7 +230,7 @@
 
 	msgb_tvlv_put(msg, BSSGP_IE_LLC_FRAMES_DISCARDED, 1, &num_frames);
 	msgb_tvlv_put(msg, BSSGP_IE_BVCI, 2, (uint8_t *) &_bvci);
-	msgb_tvlv_put(msg, BSSGP_IE_NUM_OCT_AFF, 3, (uint8_t *) &_oct_aff);
+	msgb_tvlv_put(msg, BSSGP_IE_NUM_OCT_AFF, 3, ((uint8_t *) &_oct_aff) + 1);
 
 	return gprs_ns_sendmsg(bssgp_nsi, msg);
 }