ptp_ocp: use device_find_any_child() instead of custom approach

We have already a helper to get the first child device, use it and
drop custom approach.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Vadim Fedorenko <vadfed@fb.com>
Link: https://lore.kernel.org/r/20220921141005.2443-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>