coding: use GSM_MACBLOCK_LEN gsm0503_tch_fr_decode()

Change-Id: I488ca3db1ca093d19abcfba78c1516509870715d
Related: OS#1572
diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c
index 1ef6393..3fd440f 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -1860,7 +1860,7 @@
 			return -1;
 		}
 
-		return 23;
+		return GSM_MACBLOCK_LEN;
 	}
 
 	osmo_conv_decode_ber(&gsm0503_tch_fr, cB, conv, n_errors, n_bits_total);