erasure-code: relax cauchy w restrictions

A restriction that the w parameter of the cauchy technique is limited to
8, 16 or 32 was added incorrectly while refactoring parameter parsing in
the jerasure plugin and must be relaxed.

http://tracker.ceph.com/issues/10325 Fixes: #10325

Signed-off-by: Loic Dachary <ldachary@redhat.com>