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