drivers: net: davinci_mdio: split reset function on init_clk and enable

The Davinci MDIO MDIO_CONTROL.CLKDIV can be calculated only once
during probe, hence split __davinci_mdio_reset() on
davinci_mdio_init_clk() and davinci_mdio_enable(). Initialize and
save CLKDIV in .probe(). Then just use saved value.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>