net: phy: dp83869: Set opmode from straps

If the op-mode for the device is not set in the device tree then set
the strapped op-mode and store it for later configuration.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>