fix assert for dev >= 2

[Lev91gor]

If node 0x6c connect to P2 in command:
cmn_capture.py --node 0x6c --xp 0x68 --chn 0 --samples 512

We have a problem:

Traceback (most recent call last):
  File "/home/developer/cmn-tools/src/cmn_capture.py", line 921, in <module>
    cap = ts.trace()
  File "/home/developer/cmn-tools/src/cmn_capture.py", line 802, in trace
    self.trace_start()
    ~~~~~~~~~~~~~~~~^^
  File "/home/developer/cmn-tools/src/cmn_capture.py", line 744, in trace_start
    self.configure_dtm(dtm)
    ~~~~~~~~~~~~~~~~~~^^^^^
  File "/home/developer/cmn-tools/src/cmn_capture.py", line 683, in configure_dtm
    dtm.dtm_wp_set(off, w)
    ~~~~~~~~~~~~~~^^^^^^^^
  File "/home/developer/cmn-tools/src/cmn_devmem.py", line 1186, in dtm_wp_set
    self.dtm_write64(CMN_DTM_WP0_CONFIG_off+(wp*24), w.encode())
                                                     ~~~~~~~~^^
  File "/home/developer/cmn-tools/src/cmn_devmem.py", line 967, in encode
    assert self.dev < self.xp.n_device_ports(), "%s: invalid dev_sel=%u" % (self, self.dev)
                      ^^^^^^^
AttributeError: 'DTMWatchpoint' object has no attribute 'xp'. Did you mean: 'up'?