gianfar: use DT more consistently when selecting PHY connection type

Historically, gianfar only used phy-connection-type DT property when
connected to PHY in the rgmii-id mode. It ignored the property otherwise,
relying on the connection type auto-detection carried out by MAC and
providing that reconstructed mode to of_phy_connect(). It also did not
consider alternative phy-mode property at all.

Make the driver properly query DT node for PHY connection type first and
use an obtained value if it was specified there. Otherwise, if a particular
DT relies on connection type auto-detection, fall back to reconstructing
the value from MAC registers, as before.

Signed-off-by: Arseny Solokha <asolokha@kb.kras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>