layer1: Retry fist power measurement, if it seems to fail

In some cases (e.g. after a call with TCH) the first power measurement
after a full reset will always return 0 (-110dbm). In this case the
measurment is repeated once again.

This is just a workarround, and it will not fix the actual cause.

src/target/firmware/include/layer1/sync.h

@@ -101,6 +101,7 @@ struct l1s_state {
 	struct {
 		/* power measurement l1 task */
 		unsigned int mode;
+		unsigned int first;
 		union {
 			struct {
 				uint16_t arfcn_start;

src/target/firmware/layer1/l23_api.c

@@ -419,6 +419,7 @@ static void l1ctl_rx_pm_req(struct msgb *msg)
 
 	switch (pm_req->type) {
 	case 1:
+		l1s.pm.first = 1;
 		l1s.pm.mode = 1;
 		l1s.pm.range.arfcn_start =
 				ntohs(pm_req->range.band_arfcn_from);

src/target/firmware/layer1/prim_pm.c

@@ -94,6 +94,15 @@ static int l1s_pm_resp(uint8_t num_meas, __unused uint8_t p2,
 	printf("PM MEAS: ARFCN=%u, %-4d dBm at baseband, %-4d dBm at RF\n",
 		arfcn, pm_level[0]/8, agc_inp_dbm8_by_pm(pm_level[0])/8);
 
+	/* If the first measurement seems to fail, try once again */
+	if (l1s.pm.first) {
+		l1s.pm.first = 0;
+		if (pm_level[0] == 0) {
+			l1s_pm_test(1, l1s.pm.range.arfcn_next);
+			return 0;
+		}
+	}
+
 	printd("PM MEAS: %-4d dBm, %-4d dBm ARFCN=%u\n",
 		agc_inp_dbm8_by_pm(pm_level[0])/8,
 		agc_inp_dbm8_by_pm(pm_level[1])/8, arfcn);