Fix Transponder ON Btn Display Cond to Match ON_btn_Click Definition

[divsa]

The ON_btn_Click function correctly turns Mode C off; but the check for which mode the transponder is currently in wrongly assumes that Mode S must be off. To reproduce the bug this causes,

• Connect to a transponder and set the mode to ON -> The mode text turns bold.
• Click Away from Mission Planner and the mode text will return to regular.
• This is not the case when you set the mode to be STBY or ALT.

To fix, I changed the condition check to match the definition.