[SCSI] convert SPI transport class to scsi_execute

This one's slightly more difficult.  The transport class uses
REQ_FAILFAST, so another interface (scsi_execute) had to be invented to
take the extra flag.  Also, the sense functions are shifted around to
allow spi_execute to place data directly into a struct scsi_sense_hdr.
With this change, there's probably a lot of unnecessary sense buffer
allocation going on which we can fix later.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>