Given every user of mda_vram_base expects a pointer, let
mda_vram_base be a pointer to u16.
The offset calculation in mda_detect had to be adjusted by / 2 (due to
different pointer arithmetic now).
We introduce a cast to a value returned from VGA_MAP_MEM. But I will
change VGA_MAP_MEM to return a pointer later too. But vgacon needs a
similar change first.