ath6kl: list_first_entry() is never NULL

We can remove the NULL check here.  It triggers a Smatch warning because
list_first_entry() never is NULL and people who check for it normally
intend to check for list_empty() instead.  In these cases however,
we've already verified that the lists are not empty.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>