net: fec: Adjust ENET MDIO timeouts

On extensive NFS boots on a mx6qsabrelite board it was noted that "FEC: MDIO read timeout" were occuring,
which caused failure on loading the FEC driver.

The original FEC_MII_TIMEOUT was set to 1 ms, which is too low when passed to the usecs_to_jiffies macro.

On ARM one jiffy is 10ms, so use a timeout of 30ms, which corresponds to 3 jiffies.

After running extensive NFS boots, the MDIO timeouts do not occur anymore with this change.

Signed-off-by: Rogerio Pimentel <rogerio.pimentel@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>