net: macb: fix for fixed-link mode

This patch fix the issue with fixed link. With fixed-link
device opening fails due to macb_phylink_connect not
handling fixed-link mode, in which case no MAC-PHY connection
is needed and phylink_connect return success (0), however
in current driver attempt is made to search and connect to
PHY even for fixed-link.

Fixes: 7897b071

 ("net: macb: convert to phylink")
Signed-off-by: Milind Parab <mparab@cadence.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>