ppc4xx/net: Fix MDIO clock setup

This patch fixes MDIO clock setup in case when OPB frequency is 100MHz.
Current code assumes that the value of sysinfo.freqOPB is 100000000
when OPB frequency is 100MHz. In reality it is 100000001. As a result
MDIO clock is set to incorrect value, larger than 2.5MHz, thus violating
the standard. This in not a problem on boards equipped with Marvell PHYs
(e.g. Canyonlands), since those PHYs support MDIO clocks up to 8.3MHz,
but can be a problem for other PHYs (e.g. Realtek ones).

Signed-off-by: Felix Radensky <felix@embedded-sol.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>