net: fec: setup right value for mdio hold time

The FEC modules used on i.MX28 and newer have a register to tune the MDIO
output hold time that should be at least 10 ns. Up to now this value was not
explicitly set and so resulted in less hold time if the fec clock was
faster than 100 MHz.

This was noticed on an i.MX28 machine that uses an input clock of ~150
Mhz which resulted in unreliable communication with a Marvell switch.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>