net: phy: dp83867: Use unsigned variables to store unsigned properties

The variables used to store u32 DT properties were signed ints.  This
doesn't work properly if the value of the property were to overflow.
Use unsigned variables so this doesn't happen.

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>