drivers: net: stmmac: handle start time set in the past for flexible PPS

drivers: net: stmmac: handle start time set in the past for flexible PPS

In case the time arguments used for flexible PPS signal generation are in
the past, consider the arguments to be a time offset relative to the MAC
system time.

This way, past time use case is handled and it avoids the tedious work
of passing an absolute time value for the flexible PPS signal generation
while not breaking existing scripts that may rely on this behavior.

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Link: https://patch.msgid.link/20250901-relative_flex_pps-v4-2-b874971dfe85@foss.st.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>