The script "checkpatch.pl" pointed information out like the following.

Comparison to NULL could be written "!attach"

Thus adjust this expression.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>