[SCSI] Fix scsi_add_device() for async scanning

I had thought that all drivers which didn't call scsi_scan_host()
called scsi_scan_target().  Some, such as sbp2, mptsas and libata-scsi,
call scsi_add_device() or __scsi_add_device().  We just need to wait
for the currently executing async scans to complete first.  This is the
same code that's in scsi_scan_target(), except that we have to return
an error instead of void when we're declining to scan at all.

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