Revert "mii: miiphy register address width change"

This reverts commit 5c45a22b.

It causes a lot of "incompatible pointer type" warnings for a large
number of Ethernet drivers, which are not really worth fixing
especially as this patch was only supposed to help the old,
deprecated miiphy API.  Instead of adding more efforts to a lost case
we rather revert it.

Signed-off-by: Wolfgang Denk <wd@denx.de>