apdu_split.py: Changed buf data type list to array

The data type of incoming and outgoing data should be the same
at all points of the program to make it consistent.
For this program the data type is array.array.
diff --git a/usb_application/apdu_split.py b/usb_application/apdu_split.py
index c210470..f813b8d 100755
--- a/usb_application/apdu_split.py
+++ b/usb_application/apdu_split.py
@@ -35,9 +35,9 @@
 
     def __init__(self):
         self.state = apdu_states.APDU_S_CLA
-        self.buf = []
-        self.pts_buf = []
-        self.data = []
+        self.buf = array('B', [])
+        self.pts_buf = array('B', [])
+        self.data = array('B', [])
         self.ins = array('B', [])
         self.data_remainig = 0