rtlwifi: fix a potential NULL pointer dereference

commit 76597628

 upstream.

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: Paul Gortmaker <paul.gortmaker@windriver.com>