ptp_ocp: drop duplicate NULL check in ptp_ocp_detach()

Since platform_device_unregister() is NULL-aware, we don't need to duplicate
this check. Remove it and fold the rest of the code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Vadim Fedorenko <vfedorenko@novek.ru>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>