coding: implement encoding/decoding API for TCH/F2.4

See 3GPP TS 45.003 section 3.6.  This channel mode is a bit special,
because unlike the other CSD specific channel modes it's interleaved
over 8 (not 22!) consecutive bursts, just like TCH/FS.

Change-Id: I4685376c8deb04db670684c9ebf685ad6fc989fa
Related: OS#1572
diff --git a/tests/coding/coding_test.c b/tests/coding/coding_test.c
index 7cef7b3..c9508f7 100644
--- a/tests/coding/coding_test.c
+++ b/tests/coding/coding_test.c
@@ -564,8 +564,14 @@
 		.half_rate = true,
 	},
 	{
+		.name = "TCH/F2.4",
+		.num_bits = 2 * 36,
+		.enc_fn = &gsm0503_tch_fr24_encode,
+		.dec_fn = &gsm0503_tch_fr24_decode,
+	},
+	{
 		.name = "TCH/H2.4",
-		.num_bits = 2 * 72,
+		.num_bits = 4 * 36,
 		.enc_fn = &gsm0503_tch_hr24_encode,
 		.dec_fn = &gsm0503_tch_hr24_decode,
 		.half_rate = true,