In preparation for supporting NCQ, we must allocate separate SG tables
for each command tag, rather than just a single table per port as before.

Gen-I hardware cannot do NCQ, though, so we still allocate just a single
table for that, but populate it in all 32 slots to avoid special-cases
elsewhere in hotter paths of the code.
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>