Fix inverted ChatMix level for SteelSeries Arctis 7/Pro (#475)

The chatmix level calculation had game and chat directions swapped:
max game volume produced level > 64 (indicating chat), and vice versa.

This bug originated in the old C implementation (steelseries_arctis_7.c)
and was carried over verbatim during the C++ rewrite (#443). It went
unnoticed because the old code returned a plain int with no defined
semantics for direction, users just saw a changing number. The bug
became observable once ChatmixResult defined < 64 as game and > 64 as
chat.

Author: Sapd

lib/devices/steelseries_arctis_7.hpp

@@ -155,13 +155,15 @@ class SteelSeriesArctis7 : public protocols::SteelSeriesLegacyDevice<SteelSeries
 
         // The two values are between 255 and 191
         // Convert to 0-128 range with 64 being center
+        // game==0 means game at max → level < 64 (game favored)
+        // chat==0 means chat at max → level > 64 (chat favored)
         int level;
         if (game == 0 && chat == 0) {
             level = 64;
         } else if (game == 0) {
-            level = 64 + 255 - chat;
+            level = 64 - (255 - chat);
         } else {
-            level = 64 + (-1) * (255 - game);
+            level = 64 + (255 - game);
         }
 
         // Calculate percentages (game/chat are 191-255, neutral at 255)