Author: Nils Freydank <holgersson@posteo.de>
Date:   Sat Apr 07 22:00:01 2018 +0100

Set speed_hz to a limited value as it might otherwise be default
to a system clock depending value.

--- a/pifacecommon/spi.py	2015-05-26 12:24:06.000000000 +0200
+++ b/pifacecommon/spi.py	2018-04-07 22:00:00.000000000 +0200
@@ -65,7 +65,8 @@
         transfer = spi_ioc_transfer(
             tx_buf=ctypes.addressof(wbuffer),
             rx_buf=ctypes.addressof(rbuffer),
-            len=ctypes.sizeof(wbuffer)
+            len=ctypes.sizeof(wbuffer),
+            speed_hz=ctypes.c_uint32(15000)
         )

         if self.spi_callback is not None: