vela: Minor refactor

 - Changed comments to docstring on QuantizationParams
 - Simplified op type to op name conversion

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: I2fdf5922cc17944c9bd37917a85fdfe50a1e651d
diff --git a/ethosu/vela/tflite_mapping.py b/ethosu/vela/tflite_mapping.py
index a5f7fa2..e12267f 100644
--- a/ethosu/vela/tflite_mapping.py
+++ b/ethosu/vela/tflite_mapping.py
@@ -950,13 +950,11 @@
 
 BUILTIN_OPERATOR_UNKNOWN = "UNKNOWN"
 
-
-def builtin_type_name(builtin):
-    return next(k for k, v in vars(BuiltinOperator).items() if v == builtin)
+builtin_operator_name_map = {v: k for k, v in vars(BuiltinOperator).items()}
 
 
-def optype_to_builtintype(op_type):
+def optype_to_builtintype(op_type: Op):
     if op_type in builtin_operator_inv_map:
-        return builtin_type_name(builtin_operator_inv_map[op_type][0])
+        return builtin_operator_name_map[builtin_operator_inv_map[op_type][0]]
     else:
         return BUILTIN_OPERATOR_UNKNOWN