RSL: inmplement ip.access paging load indication 'below threshold'

This is an ip.access specific 08.58 oddity.  It reports 0xffff
available paging buffers if the paging load is below the 12.21
CCCH LOAD INDICATION THRESHOLD.

We use 50, since that is what it reports if the threshold == 0.

openbsc/src/abis_rsl.c

@@ -1187,6 +1187,10 @@ static int rsl_rx_ccch_load(struct msgb *msg)
 	switch (rslh->data[0]) {
 	case RSL_IE_PAGING_LOAD:
 		pg_buf_space = rslh->data[1] << 8 | rslh->data[2];
+		if (is_ipaccess_bts(msg->trx->bts) && pg_buf_space == 0xffff) {
+			/* paging load below configured threshold, use 50 as default */
+			pg_buf_space = 50;
+		}
 		paging_update_buffer_space(msg->trx->bts, pg_buf_space);
 		break;
 	case RSL_IE_RACH_LOAD: