Fix a subtle frequency ID bookkeeping issue

[bhawkins]

In #231 we introduced a freq_band argument to the AntennaPattern constructor. The corresponding modification to focus.py script accidentally used the output frequency label instead of the one for the input file. These can be different for quasi-dual modes or mixed mode cases involving 77 MHz, which can result in an error message like

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/conda/lib/python3.12/site-packages/nisar/workflows/focus.py", line 2234, in <module>
    main(sys.argv[1:])
  File "/opt/conda/lib/python3.12/site-packages/nisar/workflows/focus.py", line 2230, in main
    focus(cfg, args.run_config_path)
  File "/opt/conda/lib/python3.12/site-packages/nisar/workflows/focus.py", line 1972, in focus
    antpat = AntennaPattern(raw, dem, antparser,
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.12/site-packages/nisar/antenna/pattern.py", line 194, in __init__
    raise ValueError(
ValueError: freq_band A is out of range ['B']!

This one-line patch just makes sure to use the subband label from the input data.

[rad-eng-59]

LGTM. Thank you for the fix.

[hfattahi]

LGTM. Thanks @bhawkins.