UBI: wl: Rename cancel flag to shutdown

It confused me more than once that the cancel flag of the
work function does not indicate the cancellation of a single work.
In fact it indicates the WL sub-system shutdown and therefore
worker functions have to free their wl_entries too.
That's why you cannot cancel a single work, you can only shutdown
all works.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>