wr_nic now exports wr_nic_pstats_callback as a pointer. When wr_pstats
is loaded it writes its own pointer in there.

wr_nic can thus ask for statistics and updated net_device_stats with
the proper numbers.   The statistics on snmp reflect this, because
snmpd uses the standard device ioctl commands -- but very few of our
counters actually end up all the way to snmp.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>