Allow waveform.set() to be called before iocInit()

With CGP-182 (944e072), waveform.set() was modified to use the dtype
attribute, which is only created after softioc.iocInit() is called.

This change adds a default for dtype, preventing this issue.

softioc/device.py

@@ -293,6 +293,7 @@ class WaveformBase(ProcessDeviceSupportCore):
     #   NELM    Length of allocated array in number of elements
     #   NORD    Currently reported length of array (0 <= NORD <= NELM)
     _fields_ = ['UDF', 'FTVL', 'BPTR', 'NELM', 'NORD']
+    dtype = None
 
     def init_record(self, record):
         self.dtype = DbfCodeToNumpy[record.FTVL]