Update udev comments re: PM342 enumeration and serial numbers

Signed-off-by: Stephen Warren <swarren@nvidia.com>

udev/swarren-lx1/99-ubtest.rules

@@ -50,7 +50,10 @@ ACTION=="add", SUBSYSTEMS=="usb", KERNELS=="3-1", ENV{UBTEST_DEVICE}="1", SYMLIN
 # which stops the script from looking for further PM342 devices. This will
 # sometimes prevent the scripts seeing a PM342, even if that specific device
 # has permissions that allow the serial number to be read. Fix this by opening
-# the permissions for all PM342 devices.
+# the permissions for all PM342 devices. Also note that all PM342 devices (and
+# in fact any FTDI devices at least with the same vid/pid) MUST have a serial
+# number defined; an empty serial number in one device also causes failures
+# finding other devices:-(
 ACTION=="add", SUBSYSTEMS=="usb", ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6011", ENV{UBTEST_DEVICE}="1"
 
 # Configure the port with permissions that will allow the test program to run