gianfar: Increase TX_TIMEOUT to 5HZ

Increased TX_TIMEOUT to 5HZ to accommodate worst case situation
for traffic and CPU intensive use cases

Signed-off-by: Priyanka Jain <Priyanka.Jain@freescale.com>
Signed-off-by: Abhimanyu <abhimanyu@freescale.com>
Acked-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>