net: sgi: ioc3-eth: Fix the size used in some 'dma_free_coherent()' calls

Update the size used in 'dma_free_coherent()' in order to match the one
used in the corresponding 'dma_alloc_coherent()'.

Fixes: 369a782a ("net: sgi: ioc3-eth: ensure tx ring is 16k aligned.")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>