[SCSI] initialize shost_data to zero

It's better to initialize host->shost_data to zero like
target->starget_data and device->sdev_data.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>