drivers/block/cciss.c: fix check-after-use

The Coverity checker spotted that we have already oops'ed if "disk"
was NULL.

Since "disk" being NULL seems impossible at this point this patch
removes the NULL check.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Acked-by: Mike Miller <mike.miller@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>