[bsc] Allow to set a EGSM900, RGSM900 ARFCN

For R-GSM900 the channels 955-1023 are within the 900er band. The
System Information Type generation can not cope with these channels yet.
diff --git a/openbsc/src/bsc_init.c b/openbsc/src/bsc_init.c
index 622fb98..617d4ae 100644
--- a/openbsc/src/bsc_init.c
+++ b/openbsc/src/bsc_init.c
@@ -802,8 +802,10 @@
 		}
 		break;
 	case GSM_BAND_900:
-		if (bts->c0->arfcn < 1 || bts->c0->arfcn > 124) {
-			LOGP(DNM, LOGL_ERROR, "GSM900 channel must be between 1-124.\n");
+		if (bts->c0->arfcn < 1 ||
+		   (bts->c0->arfcn > 124 && bts->c0->arfcn < 955) ||
+		    bts->c0->arfcn > 1023)  {
+			LOGP(DNM, LOGL_ERROR, "GSM900 channel must be between 1-124, 955-1023.\n");
 			return -EINVAL;
 		}
 		break;