net: eth: altera: do not free array priv->mdio->irq

priv->mdio->irq used to be allocated and required freeing, but it
is now a fixed sized array and should no longer be free'd.

Issue detected using static analysis with CoverityScan

Fixes: e7f4dc35

 ("mdio: Move allocation of interrupts into core")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>