rtlwifi: fix a potential NULL pointer dereference

[ Upstream commit 765976285a8c8db3f0eb7f033829a899d0c2786e ]

In case alloc_workqueue fails, the fix reports the error and
returns 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>