Made irq handling more portable (use of gpio_to_irq())

With this change the ads7846_device.c also compiles for the Beaglebone linux 3.8.13 kernel.

After modifying this file I was able to get a HY28A display up and running (Graphics and Touchscreen) with a Beaglebone Black and without any visible problems (I had a pixel problem, but it was gone after turning the cursor_blink off with the following command)
```
echo 0 > /sys/class/graphics/fbcon/cursor_blink
```