r8169: make room for more specific 8168 hardware start procedure

Broadly speaking the 8168c* share some common code which will
be factored in __rtl_hw_start_8168cp. The 8168b* share some
code too but it will be a bit different.

Any change of behavior should be confined to the currently
unidentified 8168 chipsets. They will not be applied the Tx
performance tweak and will emit a warning instead.

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