ctrl/abis: When the max_power_reduction changes, send it to the BTS

In case the max_power_reduction changes, issue a new Set Radio
Carrier Attributes command. OML 12.21 allows to not include the
ARFCN list and the semantic I picked/understand is that a partial
update is possible.

Fixes: SYS#267
diff --git a/openbsc/src/libbsc/abis_nm.c b/openbsc/src/libbsc/abis_nm.c
index f491a0c..5a96849 100644
--- a/openbsc/src/libbsc/abis_nm.c
+++ b/openbsc/src/libbsc/abis_nm.c
@@ -1466,6 +1466,12 @@
 	return abis_nm_sendmsg(trx->bts, msg);
 }
 
+int abis_nm_update_max_power_red(struct gsm_bts_trx *trx)
+{
+	uint8_t attr[] = { NM_ATT_RF_MAXPOWR_R, trx->max_power_red / 2 };
+	return abis_nm_set_radio_attr(trx, attr, ARRAY_SIZE(attr));
+}
+
 static int verify_chan_comb(struct gsm_bts_trx_ts *ts, uint8_t chan_comb,
 			const char **reason)
 {