video: fbdev: w100fb: fix sparse warnings

* Add missing __iomem annotations where needed.
* Make w100fb_probe() static.
* Return NULL pointer (instead of using plain integer) in
  w100_get_xtal_tabl().

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/d438108a-e569-a14a-a9b1-3fefd88fcadc@samsung.com