chan_alloc: Fall-back to TCH/H, if we cannot find a TCH/F

I'm not entirely sure if this is the best approach.  However,
there are phones that send a RACH request for TCH/F on MO calls, even
though they actually do support TCH/H channels.

openbsc/src/libbsc/chan_alloc.c

@@ -260,6 +260,11 @@ struct gsm_lchan *lchan_alloc(struct gsm_bts *bts, enum gsm_chan_t type,
 		break;
 	case GSM_LCHAN_TCH_F:
 		lchan = _lc_find_bts(bts, GSM_PCHAN_TCH_F);
+		/* If we don't have TCH/F available, fall-back to TCH/H */
+		if (!lchan) {
+			lchan = _lc_find_bts(bts, GSM_PCHAN_TCH_H);
+			type = GSM_LCHAN_TCH_H;
+		}
 		break;
 	case GSM_LCHAN_TCH_H:
 		lchan =_lc_find_bts(bts, GSM_PCHAN_TCH_H);