net: cw1200: fix a NULL pointer dereference

[ Upstream commit 0ed2a005347400500a39ea7c7318f1fea57fb3ca ]

In case create_singlethread_workqueue fails, the fix free the
hardware and returns NULL to avoid NULL pointer dereference.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>