ibmveth: Fix endian issues with rx_no_buffer statistic

Hidden away in the last 8 bytes of the buffer_list page is a solitary
statistic. It needs to be byte swapped or else ethtool -S will
produce numbers that terrify the user.

Since we do this in multiple places, create a helper function with a
comment explaining what is going on.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: stable@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>