hfpmodem: Handle repeated held call indicator

An issue with iPhone 5C iOS 9.2 triggers desynchronization in call
states. When an active call is put on hold and another call arrives,
it is in WAITING state. It should be possible to answer it by issuing
AT+CHLD=2 but the phone changes its state to INCOMING so ATA should be
used. This change is advertised by sending callheld:2 event, but it is
not handled. This event can be used to trigger CLCC poll to synchronize
call states.

+CIEV: 3,1   <- first call arrives
AT+CLCC
+CLCC: 1,1,4,0,0,"01234567890",129
OK
RING
+CLIP: "01234567890",129
ATA
OK
+CIEV: 2,1
+CIEV: 3,0.
AT+CHLD=2.$  <- first call is put on hold
OK
+CIEV: 7,2   <- notification confirming that call #1 is on hold
+CCWA: "09876543210",129,1     <- second call arrives
+CIEV: 7,2
+CIEV: 3,1
AT+CLCC
+CLCC: 1,1,1,0,0,"01234567890",129
+CLCC: 2,1,5,0,0,"09876543210",129 <- new call is still in WAITING state
OK
+CIEV: 7,2 <- phone iternally promotes WAITING call to INCOMING
AT+CHLD=2  <- there is no WAITING call anymore, ATA should be used
+CME ERROR:3

drivers/hfpmodem/voicecall.c

@@ -1110,6 +1110,17 @@ static void ciev_callheld_notify(struct ofono_voicecall *vc,
 			 */
 			vd->clcc_source = g_timeout_add(POLL_CLCC_DELAY,
 							poll_clcc, vc);
+		} else {
+			if (vd->clcc_source)
+				g_source_remove(vd->clcc_source);
+
+			/*
+			 * We got a notification that there is a held call
+			 * and no active call but we already are in such state.
+			 * Let's schedule a poll to see what happened.
+			 */
+			vd->clcc_source = g_timeout_add(POLL_CLCC_DELAY,
+							poll_clcc, vc);
 		}
 	}