stifb: check fb_alloc_cmap return value and handle failure properly

Signed-off-by: Andres Salomon <dilinger@debian.org>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>