[SCSI] zfcp: clear boxed flag on unit reopen.

The boxed flag for units was never cleared. This doesn't hurt, but on
ACL updates the error recovery could reopen more units than needed.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>