Initialize ktime_t type to S64_MAX (not U64_MAX)

[karlkinsella]

Using the ioctl PTP_SYS_OFFSET_EXTENDED via testptp:

#testptp -d /dev/ptp2 -x 1
extended timestamp request returned 1 samples
sample # 0: real time before: 1765392269.485510129
            phc time: 1765392306.482641995
            real time after: 1765392265.190545816       <- This value is incorrect

The issue was that the ts_window_adjust is a negative value.

#cat /sys/class/timecard/ocp0/ts_window_adjust
-3

Due to delay in ptp_ocp_estimate_pci_timing() getting initialized to a U64_MAX should be S64_MAX (as ktime_t is "typedef s64 ktime_t;")
With this change:

# cat /sys/class/timecard/ocp0/ts_window_adjust
267
# /root/testptp/testptp -d /dev/ptp2 -x 1
extended timestamp request returned 1 samples
sample # 0: real time before: 1765392342.651265647
            phc time: 1765392379.647310186
            real time after: 1765392342.651268356