Fix filter_size setter bug

self._filter was incorrectly being set with the value of _BME680_FILTERSIZES at the index `size` instead of the index of that size in the sizes list.

Author: tayden

adafruit_bme680.py

@@ -188,7 +188,7 @@ def filter_size(self):
     @filter_size.setter
     def filter_size(self, size):
         if size in _BME680_FILTERSIZES:
-            self._filter = _BME680_FILTERSIZES[size]
+            self._filter = _BME680_FILTERSIZES.index(size)
         else:
             raise RuntimeError("Invalid size")