Fixed compiler error when PS_ON_PIN is disabled

When the the *PS_ON_PIN* is disabled (i.e. defined as a negative value in pins.h e.g. there is no AUX power supply; to remove the entry from menu) the compiler throws 
```error: pasting "DIO" and "-" does not give a valid preprocessing token```
because it still checks the *SET_OUTPUT(PS_ON_PIN);* function in ui.cpp and throws the error as the *PS_ON_PIN* is defined as unsigned char.

Adding the *PS_ON_PIN* evaluation in the case statement helps to avoid this problem.