Fixed closing of PDCH when enabling sysmo-bts' direct DSP access

If not compiled with this support, the closing function does not exists,
so it must not be called.

Removed a "#warning", because closing is already supported.
diff --git a/src/sysmo_sock.cpp b/src/sysmo_sock.cpp
index 1982472..a4cc6de 100644
--- a/src/sysmo_sock.cpp
+++ b/src/sysmo_sock.cpp
@@ -99,10 +99,12 @@
 
 	/* disable all slots, kick all TBFs */
 	for (trx = 0; trx < 8; trx++) {
+#ifdef ENABLE_SYSMODSP
 		if (bts->trx[trx].fl1h) {
 			l1if_close_pdch(bts->trx[trx].fl1h);
 			bts->trx[trx].fl1h = NULL;
 		}
+#endif
 		for (ts = 0; ts < 8; ts++)
 			bts->trx[trx].pdch[ts].enable = 0;
 		for (tfi = 0; tfi < 32; tfi++) {