drivers: eth: gmac: Fixed ethernet startup when fixed-link

When upgrading to version 3.4, our hardware lost ethernet connectivity.
Our hardware assumes a fixed link and does not communicate with the
underlying phy via the mdio bus.

I confirmed that the atsame54_xpro board also will lose ethernet
functionality when the atsame54_xpro configures the phy via to use a
fixed link...

The eth_sam_gmac driver changed the initialization behavior to call
net_if_carrier_off and notes to wait until phy link is up (via
callback.)  However, when in a fixed link configuration, the callback is
never called. So the net_if_carrier_on event never occurs.

This patch adds a check to see if link is up already before calling
net_if_carrior_off. This check works because in fixed-link mode, link-up
is set synchronously during phy driver initialization.

I tested that atsame54_xpro with fixed-link configuration will now work
after this patch.

Signed-off-by: Thomas Chiantia <thomas.chiantia@gmail.com>