r8169: add KERN_DEBUG to dprintk (trivial)

- prefix dprintk with KERN_DEBUG
- fix a bug with existing use of dprintk (PFX KERN_INFO PFX)

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>