r8169: disable tx interrupt coalescing on RTL8168

In contrast to switching rx irq coalescing off what fixed an issue,
switching tx irq coalescing off is merely a latency optimization,
therefore net-next. As part of this change:

- Remove INTT_0 .. INTT_3 constants, they aren't used.

- Remove comment in rtl_hw_start_8169(), we now have a detailed
  description by the code in rtl_set_coalesce().

- Due to switching irq coalescing off per default we don't need the
  initialization in rtl_hw_start_8168(). If ethtool is used to switch
  on coalescing then rtl_set_coalesce() will configure this register.

For the sake of completeness: This patch just changes the default.
Users still have the option to configure irq coalescing via ethtool.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>