nfit/ars: Introduce scrub_flags

In preparation for introducing new flags to gate whether ARS results are
stale, or poll the completion state, convert the existing flags to an
unsigned long with enumerated values. This conversion allows the flags
to be atomically updated outside of ->init_mutex.

Reviewed-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>