b44: rename b44_mii_{read, write} to b44_mdio_{read, write}_mii

The next patch will add these functions for phylib, and we should
rename the old ones before. This now indicates that these functions are
used for the mdio registers and on the mii interface.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>