drivers: net: xen-netfront: fix array initialization bug

This patch fixes the initialization of an array used in the TX
datapath that was mistakenly initialized together with the
RX datapath arrays. An out of range array access could happen
when RX and TX rings had different sizes.

Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>