Fix empty Prepare menu

[manianac]

Fixes the blank prepare menu error. When using a single hotend, there is no for loop, so having a break statement would exit the function. While not great coding practice, its alright to loop all the hotends to check for heat.

The other option would be to add a

#if HOTENDS > 1
 break;
#endif

but I'll leave that up to @thinkyhead or whoever merges this.

I also renamed e to the index macro.

[Blue-Marlin]

@thinkyhead
Welcome back.