Do not let gpsd_serial_open() try to open a /dev/pps

For some reason the ppsthread no longer tries to open /dev/pps0,  Either
before or after this patch.

Here is the current status:

root@raspberrypi:/usr/local/src/GPS/gpsd/gpsd# gpsd -nND 6 /dev/pps0 /dev/ttyAMA0 |& fgrep pps0
gpsd:INFO: stashing device /dev/pps0 at slot 0
gpsd:ERROR: initial GPS device /dev/pps0 open failed
gpsd:INFO: KPPS:/dev/ttyAMA0 checking /sys/devices/virtual/pps/pps0/path,
[... no more output]

1 files changed, 6 insertions, 1 deletions

diff --git a/serial.c b/serial.c
index 28f65b2e..adbb3834 100644
--- a/serial.c
+++ b/serial.c
@@ -416,13 +416,18 @@ void gpsd_set_speed(struct gps_device_t *session,
 }
 
 int gpsd_serial_open(struct gps_device_t *session)
-/* open a device for access to its data */
+/* open a device for access to its data
+ * return the fd of the open device, or -1 for failure, -2 is a PPS device */
 {
     mode_t mode = (mode_t) O_RDWR;
 
     session->sourcetype = gpsd_classify(session->gpsdata.dev.path);
     session->servicetype = service_sensor;
 
+    if ( source_pps == session->sourcetype ) {
+	return -2;
+    }
+
     /*@ -boolops -type @*/
     if (session->context->readonly
 	|| (session->sourcetype <= source_blockdev)) {