kernel/wr_pistats: bugfix: read number of counters before registering

Otherwise, the port files are empty, even if pstats/info is correct

  wrs-192.168.16.9#wc -c /proc/sys/pstats/*
      529 /proc/sys/pstats/description
        7 /proc/sys/pstats/info
        0 /proc/sys/pstats/port0
        0 /proc/sys/pstats/port1
        0 /proc/sys/pstats/port10
        0 /proc/sys/pstats/port11
        0 /proc/sys/pstats/port12
        0 /proc/sys/pstats/port13
        0 /proc/sys/pstats/port14
        0 /proc/sys/pstats/port15
        0 /proc/sys/pstats/port16
        0 /proc/sys/pstats/port17
        0 /proc/sys/pstats/port2
        0 /proc/sys/pstats/port3
        0 /proc/sys/pstats/port4
        0 /proc/sys/pstats/port5
        0 /proc/sys/pstats/port6
        0 /proc/sys/pstats/port7
        0 /proc/sys/pstats/port8
        0 /proc/sys/pstats/port9
      536 total

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>