If we're trying to allocate TCH/H but don't have it, fallback to TCH/F

The rationale is as following:

If we have NECI=1, then the phone will request a channel with CHREQ
"0100xxxx Originating speech call from dual-rate mobile station when TCH/H is
 sufficient and supported by the MS for speech calls", then we will try to
allocate a TCH/H [as it is sufficient].

However, if there are no free TCH/H slots on the BTS, we abandon and can't
handle the MO call at all :(

openbsc/src/chan_alloc.c

@@ -203,6 +203,9 @@ struct gsm_lchan *lchan_alloc(struct gsm_bts *bts, enum gsm_chan_t type)
 		break;
 	case GSM_LCHAN_TCH_H:
 		lchan =_lc_find_bts(bts, GSM_PCHAN_TCH_H);
+		/* If we don't have TCH/H available, fall-back to TCH/F */
+		if (!lchan)
+			lchan = _lc_find_bts(bts, GSM_PCHAN_TCH_F);
 		break;
 	default:
 		fprintf(stderr, "Unknown gsm_chan_t %u\n", type);