dca: remove unneeded NULL check

The return here doesn't release the locks or re-enable IRQs.  But as
Andrew Morton points out, domain is never NULL.  list_first_entry()
essentially never returns NULL and also we already verified that the list
is not empty.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>