ARM: imx6: fix wrong fec clk

imx_get_fecclk() returns enet_ref instead of ipg.
Since the clock is used to calculate the prescaler
for the MDIO interface wrong values can be calculated.

Tested on a custom MX6S board with 100MBit interface

Signed-off-by: Markus Niebel <Markus.Niebel@tqs.de>