NET: fec_mxc: fix MDIO clock prescaler calculation

The prescaler value for MDC is calculated wrong for
MX6S. Implement the same logic as linux here which handles
ENET_MAC different then original FEC.

Tested on a custom board with i.MX6S and 100MBit PHY

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