[SCSI] scsi_transport_spi: convert to attribute groups

This conversion makes full use of the is_visible() callback on attribute
groups.  Now, each device appears only with its capability flags in the
transport class directory.  Previously each device appeared with the
capability of the host, so this is a functionality improvement.
Converting to attribute groups allows us to sweep away most of the home
grown #defines that were effectively doing the same thing.

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