s2io: resolve statistics issues

This patch resolves a number of issues in the statistics gathering of
the s2io driver.

On Xframe adapters, the received multicast statistics counter includes
pause frames which are not indicated to the driver.  This can cause
issues where the multicast packet count is higher than what has actually
been received, possibly higher than the number of packets received.

The driver software counters are replaced with the adapter hardware
statistics for rx_packets, rx_bytes, and tx_bytes.  It also uses the
overflow registers to determine if the statistics wrapped the 32bit
register (removing the window of having a statistic value less than the
previous call).  rx_length_errors statistic now includes undersized
packets in addition to oversized packets in its counting.  Finally,
rx_crc_errors are now being counted.

Signed-off-by: Jon Mason <jon.mason@exar.com>
Signed-off-by: David S. Miller <davem@davemloft.net>