Fix for PR #3502(General cleanup of arc code)

[ghost]

Fix? for PR #3502(General cleanup of arc code)

After merging of PR #3502, I got a new warning as follows.

AppData\Local\Temp\build6674025c445f92d0b07185e4e4080804.tmp\sketch\Marlin_main.cpp:7303:20: warning: statement has no effect [-Wunused-value]

     angular_travel == RADIANS(360);

                    ^

I'm not confident that this my fix is right, but at least, I think that "==" in this place isn't suitable.

...perhaps, "=" is right?

[thinkyhead]

Either too much or not enough coffee.