cifs: fix session setup unicode string saving to use new unicode helpers

...and change decode_unicode_ssetup to be a void function. It never
returns an actual error anyway.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>