This adds the final missing piece to the transport attributes:  A
published API by which they can be set and retrieved (SPI attributes
only).  The sysfs field only appears writeable if the driver supplied a
set method in the attribute template, so unsettable attributes show up
as read only.

The consequence now is that the spi transport attribute class is no
longer a simple exported structure: the driver has to attach to the spi
transport class at module initialisation.