wifi: iwl3945: Add missing check for create_singlethread_workqueue

Add the check for the return value of the create_singlethread_workqueue
in order to avoid NULL pointer dereference.

Fixes: b481de9c ("[IWLWIFI]: add iwlwifi wireless drivers")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230208063032.42763-2-jiasheng@iscas.ac.cn