[SCSI] a4000t, zorro7xx, mvme16x, bvme6000,sim710: xxx_device_remove seems buggy

Fix drivers misusing dev_to_shost

Some drivers were using dev_to_shost to go from a struct device to the
corresponding shost.  Unfortunately, dev_to_shost only looks up the tree
to find an shost (it's designed to go from a scsi_device or a
scsi_target to the parent scsi_host), and these drivers were calling it
with the parent of the scsi_host.

I've fixed this by saving a pointer to the Scsi_Host in the drvdata,
which matches what most scsi drivers do.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>