wifi: iwlwifi: Adopt PTP cross timestamps to core changes

wifi: iwlwifi: Adopt PTP cross timestamps to core changes

iwlwifi only supports CLOCK_REALTIME timestamps and provides an incomplete
result without system counter values etc.

It also zeros struct system_device_crosststamp, which is already zeroed in
the core and initialized with the clock ID.

Remove the zeroing and reject any request for a clock ID other than REALTIME.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Arthur Kiyanovski <akiyano@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20260529195557.535447186@kernel.org