net: phy: micrel: use devm_clk_get_optional_enabled for the rmii-ref clock

net: phy: micrel: use devm_clk_get_optional_enabled for the rmii-ref clock

While the external clock input will most likely be enabled, it's not
guaranteed and clk_get_rate in some suppliers will even just return
valid results when the clock is running.

So use devm_clk_get_optional_enabled to retrieve and enable the clock
in one go.

Signed-off-by: Heiko Stuebner <heiko.stuebner@cherry.de>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20231201150131.326766-2-heiko@sntech.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>