net: sxgbe: fix error paths in sxgbe_platform_probe()

We need to use post-decrement to ensure that irq_dispose_mapping is
also called on priv->rxq[0]->irq_no; moreover, if one of the above for
loops failed already at i==0 (so we reach one of these labels with
that value of i), we'll enter an essentially infinite loop of
out-of-bounds accesses.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>