ptp_ocp: replace kzalloc(x*y) by kcalloc(y, x)

While here it may be no difference, the kcalloc() has some checks
against overflow and it's logically correct to call it for an array.

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>