ibm_newemac: Correct opb_bus_freq value

The EMAC4_MR1_OBCI(freq) macro expects freg in MHz,
while opb_bus_freq is kept in Hz. Correct this.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>