Fix API parameter types for readData()

[ImpulseAdventure]

The existing readData() API has mismatched integer type widths, preventing the user from using the API to query the raw z (pressure) value.

This simple fix adjusts the API parameters to match the returned data types.

Existing API:

void XPT2046_Touchscreen::readData(uint16_t *x, uint16_t *y, uint8_t *z)
{
	update();
	*x = xraw;
	*y = yraw;
	*z = zraw;
}

Raw value declarations:

private:
	int16_t xraw=0, yraw=0, zraw=0;

Thanks