Cherry-pick for xmega: Fixing warnings (unsigned comparisons to 0). (maniacbug)
Cherry-pick for xmega: Fixing warnings (unsigned comparisons to 0). (maniacbug)

jaehong/Xmegaduino,jaehong/Xmegaduino,jaehong/Xmegaduino,jaehong/Xmegaduino,jaehong/Xmegaduino,jaehong/Xmegaduino,jaehong/Xmegaduino