net: stmmac: Fixed ethool speed issue

Don't modify emac supported capabilities during
early ethernet boot up. Only update advertise values
otherwise speed can't be changed using ethtool.

Change-Id: Id25ddd17a81fefd4d17116f0ff7237272f2607c0
Signed-off-by: Suraj Jaiswal <jsuraj@codeaurora.org>