net: qfec: Fix incorrect SMI timing

The purpose of this change is to correct the default
MDC clock frequency based on the Ethernet PHY specification.

Change-Id: I08a290b35929f031dd740951643deefb56a1b95c
Signed-off-by: Mayank Goyal <goyalm@codeaurora.org>
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>