dca: delete unnecessary variable

It's more readable to just pass NULL directly instead of using a variable
for that.

Link: https://lkml.kernel.org/r/Y/yAlDytLH0ZNLNz@kili


Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>