coding: gsm0503_pdtch_decode(): implement USF decoding for CS1

This patch brings decoding of the CS1 in consistency with the other
three coding schemes, for which we support decoding USF independently.

Change-Id: I61a3628741c0ac68374fc7f077cf3a07e51277c3
---
M src/coding/gsm0503_coding.c
1 file changed, 16 insertions(+), 0 deletions(-)

diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c
index 5b78502..1f409d6 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -1042,6 +1042,10 @@
 		osmo_conv_decode_ber(&gsm0503_xcch, cB,
 			conv, n_errors, n_bits_total);
 
+		/* the three USF bits d(0),d(1),d(2) are *not* precoded */
+		if (usf_p)
+			*usf_p = (conv[0] << 2) | (conv[1] << 1) | (conv[2] << 0);
+
 		rv = osmo_crc64gen_check_bits(&gsm0503_fire_crc40,
 			conv, 184, conv + 184);
 		if (rv)

