Fix Pupil camera mode detection in vedc generate_config

[phausamann]

As reported by @csinnott91:

Traceback (most recent call last):
  File "/home/sml04/vedb/miniconda3/envs/vedc/bin/vedc", line 33, in <module>
    sys.exit(load_entry_point('ved-capture', 'console_scripts', 'vedc')())
  File "/home/sml04/vedb/miniconda3/envs/vedc/lib/python3.6/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/sml04/vedb/miniconda3/envs/vedc/lib/python3.6/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/home/sml04/vedb/miniconda3/envs/vedc/lib/python3.6/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/sml04/vedb/miniconda3/envs/vedc/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/sml04/vedb/miniconda3/envs/vedc/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/home/sml04/vedb/ved-capture/ved_capture/cli/config.py", line 125, in generate_config
    config = get_uvc_config(config, device_name, uid)
  File "/home/sml04/vedb/ved-capture/ved_capture/cli/utils.py", line 201, in get_uvc_config
    pri.VideoDeviceUVC._get_available_modes(uid)
AttributeError: type object 'VideoDeviceUVC' has no attribute '_get_available_modes'

This is due to the refactored VideoDeviceUVC interface (and a good lesson why it's a bad idea to use private methods from other packages).