[SCSI] aic7xxx: expose the bus setting to sysfs

Read the transciever register and display in the host transport
properties.  I'm still not entirely sure what this does for multiple
transciever adapters (like some 160 ones) however, I suspect it
displays the transciever state of the switchable bus segment.

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