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>

drivers/dca/dca-core.c

@@ -294,9 +294,7 @@ EXPORT_SYMBOL_GPL(dca3_get_tag);
  */
 u8 dca_get_tag(int cpu)
 {
-	struct device *dev = NULL;
-
-	return dca_common_get_tag(dev, cpu);
+	return dca_common_get_tag(NULL, cpu);
 }
 EXPORT_SYMBOL_GPL(dca_get_tag);