multi-ARFCN: fix maximum number of carriers limitation

Maximum number of carriers is fixed to 3 channels on a single
physical RF channel in multi-ARFCN mode. For some reason, it
was limited to 5.

Let's fix this, and also follow this limitation in the
following VTY command handlers:

  - cfg_multi_arfcn_cmd,
  - cfg_chan_cmd.

Change-Id: I66a1462f368458afd313ee6f0bc0abc496dde817
diff --git a/Transceiver52M/osmo-trx.cpp b/Transceiver52M/osmo-trx.cpp
index b252d0c..7489e7c 100644
--- a/Transceiver52M/osmo-trx.cpp
+++ b/Transceiver52M/osmo-trx.cpp
@@ -411,7 +411,7 @@
 
 int trx_validate_config(struct trx_ctx *trx)
 {
-	if (trx->cfg.multi_arfcn && trx->cfg.num_chans > 5) {
+	if (trx->cfg.multi_arfcn && trx->cfg.num_chans > TRX_MCHAN_MAX) {
 		LOG(ERROR) << "Unsupported number of channels";
 		return -1;
 	}