net: mdio: mux-meson-g12a: use __clk_is_enabled to simplify the code

By using __clk_is_enabled () we can avoid defining an own variable for
tracking whether enable counter is zero.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>