Fixed wrong bitwise operation instruction

For example, `or c` used to modify the c register,
but it should store the results in the a register
instead

src/cpu/opcodes.rs

@@ -134,16 +134,14 @@ impl GameBoy {
 // Bitwise operation functions
 impl GameBoy {
     fn bitwise_operation_r(&mut self, register: OneByteRegister, operation: BitwiseOperation) {
-        let register_a = *self.registers.get_r(OneByteRegister::A);
-        let register = self.registers.get_r(register);
+        let register = *self.registers.get_r(register);
 
         let result = match operation {
-            BitwiseOperation::Or => *register | register_a,
-            BitwiseOperation::Xor => *register ^ register_a,
+            BitwiseOperation::Or => register | self.registers.a,
+            BitwiseOperation::Xor => register ^ self.registers.a,
         };
 
-        *register = result;
-
+        self.registers.a = result;
         self.flags.update_zero_flag(result);
     }
 }