net: ag71xx: Slighly simplify code in 'ag71xx_rings_init()'

A few lines above, we have:
   tx_size = BIT(tx->order);

So use 'tx_size' directly to be consistent with the way 'rx->descs_cpu' and
'rx->descs_dma' are computed below.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>