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: f5e1d0d ("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>

Author: Markus Blöchl

drivers/net/ethernet/stmicro/stmmac/dwmac-intel.c

@@ -433,6 +433,12 @@ static int intel_crosststamp(ktime_t *device,
 		return -ETIMEDOUT;
 	}
 
+	*system = (struct system_counterval_t) {
+		.cycles = 0,
+		.cs_id = CSID_X86_ART,
+		.use_nsecs = false,
+	};
+
 	num_snapshot = (readl(ioaddr + GMAC_TIMESTAMP_STATUS) &
 			GMAC_TIMESTAMP_ATSNS_MASK) >>
 			GMAC_TIMESTAMP_ATSNS_SHIFT;
@@ -448,7 +454,7 @@ static int intel_crosststamp(ktime_t *device,
 	}
 
 	system->cycles *= intel_priv->crossts_adj;
-	system->cs_id = CSID_X86_ART;
+
 	priv->plat->flags &= ~STMMAC_FLAG_INT_SNAPSHOT_EN;
 
 	return 0;