libata: add extra internal command

Bump the internal tag to 32, instead of stealing the last tag in
our regular command space. This works just fine, since we don't
actually need a separate hardware tag for this. Internal commands
cannot coexist with NCQ commands.

As a bonus, we get rid of the special casing of what tag to use
for the internal command.

This is in preparation for utilizing all 32 commands for normal IO.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Tejun Heo <tj@kernel.org>