tools: ynl: fix get_mask utility routine

Fix get_mask utility routine in order to take into account possible gaps
in the elements list.

Fixes: be5bea1 ("net: add basic C code generators for Netlink")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/nlspec.py

@@ -138,10 +138,8 @@ def has_doc(self):
 
     def get_mask(self):
         mask = 0
-        idx = self.yaml.get('value-start', 0)
-        for _ in self.entries.values():
-            mask |= 1 << idx
-            idx += 1
+        for e in self.entries.values():
+            mask += e.user_value()
         return mask