Convert Encoder callbacks to be boolean functions (#12805)

Co-authored-by: James Young <[email protected]>

quantum/encoder.h

@@ -22,8 +22,8 @@
 void encoder_init(void);
 bool encoder_read(void);
 
-void encoder_update_kb(int8_t index, bool clockwise);
-void encoder_update_user(int8_t index, bool clockwise);
+bool encoder_update_kb(uint8_t index, bool clockwise);
+bool encoder_update_user(uint8_t index, bool clockwise);
 
 #ifdef SPLIT_KEYBOARD
 void encoder_state_raw(uint8_t* slave_state);