net: spider_net: Fix the size used in a 'dma_free_coherent()' call

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

Fixes: d4ed8f8d ("Spidernet DMA coalescing")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>