serial.c: Some ports don't support TIOCMBIS for RTS/CTS control

So let's ignore the error we might get from this opreration.

Change-Id: I69dad90403355b6512c3bf4c7fd10dd1dd985f01
diff --git a/src/serial.c b/src/serial.c
index 4403226..1cbd3c0 100644
--- a/src/serial.c
+++ b/src/serial.c
@@ -113,8 +113,8 @@
 	rc = ioctl(fd, TIOCMBIS, &v24);
 	if (rc < 0) {
 		dbg_perror("ioctl(TIOCMBIS)");
-		rc = -errno;
-		goto error;
+		/* some serial porst don't support this, so let's not
+		 * return an error here */
 	}
 
 	return fd;