tools/net/ynl: ethtool: fix crash when Hardware Clock info is missing

tools/net/ynl: ethtool: fix crash when Hardware Clock info is missing

Fix a crash in the ethtool YNL implementation when Hardware Clock information
is not present in the response. This ensures graceful handling of devices or
drivers that do not provide this optional field. e.g.

  Traceback (most recent call last):
    File "/net/tools/net/ynl/pyynl/./ethtool.py", line 438, in <module>
      main()
      ~~~~^^
    File "/net/tools/net/ynl/pyynl/./ethtool.py", line 341, in main
      print(f'PTP Hardware Clock: {tsinfo["phc-index"]}')
                                   ~~~~~~^^^^^^^^^^^^^
  KeyError: 'phc-index'

Fixes: f3d07b02b2b8 ("tools: ynl: ethtool testing tool")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250508035414.82974-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>