AVR32: Fix logic inversion in disable_interrupts()

disable_interrupts() should return nonzero if interrupts were
_enabled_ before, not disabled.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

lib_avr32/interrupts.c

@@ -35,5 +35,5 @@ int disable_interrupts(void)
 	sr = sysreg_read(SR);
 	asm volatile("ssrf	%0" : : "n"(SYSREG_GM_OFFSET));
 
-	return SYSREG_BFEXT(GM, sr);
+	return !SYSREG_BFEXT(GM, sr);
 }