Incorrect emulation of SLOT-2 GPIO

[edo9300]

Currently melon handles the SLOT-2 GPIO by returning 0 whenever the 3 mapped addresses are attempted to be read if GPIO is disabled,

melonDS/src/GBACart.cpp

Lines 234 to 247 in 15c3faa

	if (addr >= 0xC4 && addr < 0xCA)
	{
	if (GPIO.control & 0x1)
	{
	switch (addr)
	{
	case 0xC4: return GPIO.data;
	case 0xC6: return GPIO.direction;
	case 0xC8: return GPIO.control;
	}
	}
	else
	return 0;
	}

This doesn't match hardware where reads to those addresses correctly return the value stored in the card's rom if the specific card doesn't have the addresses matched to a gpio interface (for example a generic flashable cart or a supercard flashed with a custom firmware running in ds mode).
It should be investigated if the behaviour is entirely on the game pack itself or there's something else being done by the ds itself