ptp_ocp: use bits.h macros for all masks

Currently we are using BIT(), but GENMASK(). Make use of the latter one
as well (far less error-prone, far more concise).

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>