mdacon: make mda_vram_base u16 *

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.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>