mii: Remove references to DP83840 PHY in mii.h

There are references to this PHY chip in the generic mii.h header, so removing them.
Re-jiggle the changed comments, in response to points raised by Ben Hutchings.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>