[SCSI] fix missing check for no scanning

Drivers that called scsi_scan_target() instead of scsi_scan_host() were
still adding devices; this needs to be under the control of userspace,
not the driver.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>