ionic: fix mem leak in rx_empty

The sentinel descriptor entry was getting missed in the
traverse of the ring from head to tail, so change to a
loop of 0 to the end.

Fixes: f1d2e894 ("ionic: use index not pointer for queue tracking")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>