Change the sa(4) driver to check for long position support on

Change the sa(4) driver to check for long position support on
SCSI-2 devices.

Some older tape devices claim to be SCSI-2, but actually do support
long position information.  (Long position information includes
the current file mark.)  For example, the COMPAQ SuperDLT1.

So we now only disable the check on SCSI-1 and older devices.

sys/cam/scsi/scsi_sa.c:
	In saregister(), only disable fetching long position
	information on SCSI-1 and older drives.  Update the
	comment to explain why.

Confirmed by:	dvl
Sponsored by:	Spectra Logic
MFC after:	3 weeks