mbxfb: fix incorrect argument type

Fix wrong pointer type passed into the dev_dbg() function.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Acked-by: Mike Rapoport <mike@compulab.co.il>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>