rtlwifi: fix a potential NULL pointer dereference

rtlwifi: fix a potential NULL pointer dereference

BugLink: https://bugs.launchpad.net/bugs/1837517
[ 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>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>