miiphy: leverage current_mii cache more

For code that uses miiphy_{read,write}, every call invokes a full look up
of the mii list.  There is already a "current_mii" cache that is used by
some code, but have the miiphy_{read,write} function use it as well.  This
does increase the code size slightly, but I think it's worth it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>