davinci_mdio: Correct bitmask for clock divider value

The CLKDIV bitfield in the MDIO Control Register is a 16 bit field,
therefore the CLKDIV value may range from 0 to 0xffff.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>