net/ariadne: Improve debug prints

Remove casts and use proper printf()-style format specifiers instead.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Acked-by: David S. Miller <davem@davemloft.net>