net/fsl: remove hardcoded clock setting from xgmac_mdio

There is no need to set the clock speed in read/write which will be performed
unnecessarily for each mdio access. Init it during probe is enough.

Also, the hardcoded clock value is not a proper way for all SoCs.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>