net: mdio: mdio-bcm-unimac: Correct rate fallback logic

net: mdio: mdio-bcm-unimac: Correct rate fallback logic

When the parent clock is a gated clock which has multiple parents, the
clock provider (clk-scmi typically) might return a rate of 0 since there
is not one of those particular parent clocks that should be chosen for
returning a rate. Prior to ee975351cf0c ("net: mdio: mdio-bcm-unimac:
Manage clock around I/O accesses"), we would not always be passing a
clock reference depending upon how mdio-bcm-unimac was instantiated. In
that case, we would take the fallback path where the rate is hard coded
to 250MHz.

Make sure that we still fallback to using a fixed rate for the divider
calculation, otherwise we simply ignore the desired MDIO bus clock
frequency which can prevent us from interfacing with Ethernet PHYs
properly.

Fixes: ee975351cf0c ("net: mdio: mdio-bcm-unimac: Manage clock around I/O accesses")
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250730202533.3463529-1-florian.fainelli@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>