r8169: cleanup

No functionnal change:
- trim the old history log
- whitespace/indent/case police
- unsigned int where signedness does not matter
- removal of obsolete assert
- needless cast from void * (dev_instance)
- remove dead code once related to power management
- use netdev_alloc_skb.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Stephen Hemminger <shemminger@linux-foundation.org>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>