drivers: ethernet: stm32: Bugfix PTP clock read on second boundary

Current version of STM32 PTP clock reads current PTP time by querying
second and nanosecond registers sequentially. It is possible for second
to roll over between reading second and nanosecond registers, causing
returned time to be off by a second. This bugfix resolves that issue.

Signed-off-by: Alex Sergeev <asergeev@carbonrobotics.com>