Currnetly the ->bios_param for host drivers exposes struct scsi_disk
(aka Scsi_Disk or Disk) to each and every lowlevel driver, although
this structure should be privated to the sd driver.

All bios_param implementation do only use two fields:  .device and
.capacity.  This patch passes down those two directly and gets
rid of 99% of the sd.h inclusions (*).

I've tried to not break any driver with this patch, but given the
number of compiler errors in the current tree I might have missed
one or two.

(*) a bunch of drivers needed sd.h to get to scsi.h, I've fixed those.