lan743x: replace devicetree phy parse code with library function

The code in this driver which parses the devicetree to determine
the phy/fixed link setup, can be replaced by a single library
function: of_phy_get_and_connect().

Behaviour is identical, except that the library function will
complain when 'phy-connection-type' is omitted, instead of
blindly using PHY_INTERFACE_MODE_NA, which would result in an
invalid phy configuration.

The library function no longer brings out the exact phy_mode,
but the driver doesn't need this, because phy_interface_is_rgmii()
queries the phydev directly. Remove 'phy_mode' from the private
adapter struct.

While we're here, log info about the attached phy on connect,
this is useful because the phy type and connection method is now
fully configurable via the devicetree.

Tested on a lan7430 chip with built-in phy. Verified that adding
fixed-link/phy-connection-type in the devicetree results in a
fixed-link setup. Used ethtool to verify that the devicetree
settings are used.

Tested-by: Sven Van Asbroeck <thesven73@gmail.com> # lan7430
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201116170155.26967-1-TheSven73@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>