net: stmmac: intel: populate entire system_counterval_t in get_time_fn() callback

net: stmmac: intel: populate entire system_counterval_t in get_time_fn() callback

get_time_fn() callback implementations are expected to fill out the
entire system_counterval_t struct as it may be initially uninitialized.

This broke with the removal of convert_art_to_tsc() helper functions
which left use_nsecs uninitialized.

Initially assign the entire struct with default values.

Fixes: f5e1d0db3f02 ("stmmac: intel: Remove convert_art_to_tsc()")
Cc: stable@vger.kernel.org
Signed-off-by: Markus Blöchl <markus@blochl.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250713-stmmac_crossts-v1-1-31bfe051b5cb@blochl.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>