r8169: add hooks for per-device hw_start handler

Rationale: rtl8169_hw_start will not help maintaining an unified
driver for different chipsets but people at Realtek are probably
too polite to say it distinctly.

Let's add the hook and keep hw_start handler unchanged.

As can be seen from the content of rtl8169_pci_tbl, the RTL_CFG_1
entry in rtl_cfg_info was unused. I recycled it for the 0x8168.

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