rsl: Support PDCH in rsl_chan_nr_str

During log review lack of support  for PDCH was spotted:

scheduler.c:593 Configuring multiframe with PDCH trx=0 ts=4
rsl.c:1159 (bts=0,trx=0,ts=4,ss=0): chan_nr=UNKNOWN on TS4 type=0x00 mode=SIGNALLING
rsl.c:741 (bts=0,trx=0,ts=4,pchan=TCH/F_TCH/H_PDCH as PDCH) (ss=0) PDTCH Tx CHAN ACT ACK

Wireshark shows:
1100 0... = C-bits: PDCH (24)
.... .100 = Time slot number (TN): 4

Change-Id: If6ed96c9eb0431eb24e2135634def61e0ea506ea
diff --git a/src/gsm/rsl.c b/src/gsm/rsl.c
index abda932..e610ebf 100644
--- a/src/gsm/rsl.c
+++ b/src/gsm/rsl.c
@@ -236,6 +236,8 @@
 		sprintf(str, "RACH on TS%d", ts);
 	else if (cbits == 0x12)
 		sprintf(str, "PCH/AGCH on TS%d", ts);
+	else if (cbits == 0x18)
+		sprintf(str, "PDCH on TS%d", ts);
 	else
 		sprintf(str, "UNKNOWN on TS%d", ts);