staging: nvec: use system_nrt_wq instead of custom one

There isn't much reason to use custom workqueue in nvec.  It can use
system_nrt_wq instead and cancel the two work items on removal.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>