Reduce RTL SDR buffers size

In memory limited devices like RPi, the default buffer allocation scheme
of the RTL driver causes problems to the async transfer engine.
This prevented the flowgraphs to start. 
Reducing the buffer size fixes this problem.

python/hw_settings.py

@@ -2,7 +2,8 @@
 #
 # gr-satnogs: SatNOGS GNU Radio Out-Of-Tree Module
 #
-#  Copyright (C) 2016, Libre Space Foundation <http://librespacefoundation.org/>
+#  Copyright (C) 2016,2017
+#  Libre Space Foundation <http://librespacefoundation.org/>
 #
 #  This program is free software: you can redistribute it and/or modify
 #  it under the terms of the GNU General Public License as published by
@@ -47,4 +48,4 @@ hw_rx_settings = {'usrpb200' : {'rf_gain' : 50.0, 'if_gain' : 0.0,
                               'antenna' : '', 'dev_arg': 'hackrf'},
                   'rtlsdr' : {'rf_gain' : 32.8, 'if_gain' : 0.0,
                               'bb_gain' : 0.0, 'samp_rate' : 1e6,
-                              'antenna' : '', 'dev_arg' : 'rtl'} }
+                              'antenna' : '', 'dev_arg' : 'rtl,buffers=32,buflen=16384' }}