r8169: Handle rxfifo errors on 8168 chips

The Thinkpad X100e seems to have some odd behaviour when the display is
powered off - the onboard r8169 starts generating rxfifo overflow errors.
The root cause of this has not yet been identified and may well be a
hardware design bug on the platform, but r8169 should be more resiliant to
this. This patch enables the rxfifo interrupt on 8168 devices and removes
the MAC version check in the interrupt handler, and the machine no longer
crashes when under network load while the screen turns off.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>