e1000: Fixing the received/transmitted octets' counters

e1000: Fixing the received/transmitted octets' counters

Previously, these 64-bit registers did not stick at their maximal
values when (and if) they reached them, as they should do, according to
the specs.

This patch introduces a function that takes care of such registers,
avoiding code duplication, making the relevant parts more compatible
with the QEMU coding style, while ensuring that in the unlikely case
of reaching the maximal value, the counter will stick there, as it
supposed to.

Signed-off-by: Leonid Bloch <leonid.bloch@ravellosystems.com>
Signed-off-by: Dmitry Fleytman <dmitry.fleytman@ravellosystems.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>