ethtool: init tsinfo stats if requested

ethtool: init tsinfo stats if requested

Statistic values should be set to ETHTOOL_STAT_NOT_SET even if the
device doesn't support statistics. Otherwise zeros will be returned as
if they are proper values:

host# ethtool -I -T lo
Time stamping parameters for lo:
Capabilities:
	software-transmit
	software-receive
	software-system-clock
PTP Hardware Clock: none
Hardware Transmit Timestamp Modes: none
Hardware Receive Filter Modes: none
Statistics:
  tx_pkts: 0
  tx_lost: 0
  tx_err: 0

Fixes: 0e9c127729be ("ethtool: add interface to read Tx hardware timestamping statistics")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Vadim Fedorenko <vadfed@meta.com>
Reviewed-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Link: https://lore.kernel.org/r/20240530040814.1014446-1-vadfed@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>