Update GenericComPlugin.cs

Hello CV,  
I have continued to work on my gun controller, 
and am at the point where, almost everything is working, 
but the serial data lag is still a problem.

The previous pull request based on your advice, improved the lag, 
but it lasts only for about 24 hrs, after which I must re-boot my computer to fix it again. 
But now, it seems that even that is not fixing it anymore.....

When viewing the data in the arduino serial monitor, there is no lag, 
but there is major lag  when watching it in FP (10secs+), 
so I guess the issue must be in this code.

I would like to try to add a PurgeComm method to the code, 
but I do not understand c# enough to get it to work, 
so I would really like to hear your advise..

cheers!