net: fddi: skfp: Remove addr_to_string().

kbuild test robot found that addr_to_string() is available only when
DEBUG is defined. And I found that what that function is doing is
what %pM will do. Thus, replace %s with %pM and remove thread-unsafe
addr_to_string() function.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>