Added support for Width/Height > 255 in default fbtft_set_addr_win

The current code sets only the low byte for width and height and does not work for displays that have a high byte != 0 in width or height.

With this change the Watterott display (with ILI9341) works in flexfb, too (It has a width of 320 pixels). Its fully compatible to the old version.
The change also cleans up the ili9341fb driver that does not need a custom set_addr_win anymore and uses the default one.