net: lan966x: Update lan966x_ptp_get_nominal_value

The clk_per_cfg register represents the value added to the system clock
for each clock cycle. The issue is that the default value is wrong,
meaning that in case the DUT was a grandmaster then everone in the
network was too slow. In case there was a grandmaster, then there is no
issue because the DUT will configure clk_per_cfg register based on the
master frequency.

Fixes: d0964594 ("net: lan966x: Add support for ptp clocks")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>