sfc: Reverse initial buffer table allocation to allow for later resizing

We have a very simple way of allocating buffer table entries to
queues, which is just to take the next one available.  The extra
channels are the highest numbered channels but they need to be
allocated the lowest entries so that the traffic channels can be
allocated new entries without any collisions.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>