use if_baudrate and if_link_state for mac config, not mii media values

use if_baudrate and if_link_state for mac config, not mii media values

the phy code sets if_baudrate and if_link_state, so the information
needed to config the mac on the chip is there anyway. it also has
the benefit that the driver doesnt have to understand every type
of media (eg, 1000baseTX vs 1000baseSX) because they're both the
same speed and that's what matters when configuring the chip and
the clocks etc.

this is a step toward being able to use a fixed-link node in the
device tree instead of a phy, as is found on the banana pi bpi-r2
pro on the gmac connected to a switch chip.