rtlwifi: use alloc_workqueue

create_workqueue is deprecated.  The workqueue usage does not seem to
demand any special treatment, so do not set any flags either.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Acked-by: Tejun Heo <tj@kernel.org>