common/miiphyutil.c: Cleanup MII_DEBUG and debug()

Current MII_DEBUG is confusing in two ways. One is useless define-then-
undef at the top of the file. The other is there is only one debug() in
this file, and that doesn't seem worthwhile to bother having MII_DEBUG.
While there are many useful printf()/puts() debug codes, but they are for
DEBUG, not for MII_DEBUG.

This patch tries to put them all together into MII_DEBUG and debug().

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>