be2iscsi: set the boot_kset pointer to NULL in case of failure

In beiscsi_setup_boot_info(), the boot_kset pointer should be set to
NULL in case of failure otherwise an invalid pointer dereference may
occur later.

Cc: <stable@vger.kernel.org>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Jitendra Bhivare <jitendra.bhivare@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>