igc: Handle PPS start time programming for past time values

I225/6 hardware can be programmed to start PPS output once
the time in Target Time registers is reached. The time
programmed in these registers should always be into future.
Only then PPS output is triggered when SYSTIM register
reaches the programmed value. There are two modes in i225/6
hardware to program PPS, pulse and clock mode.

There were issues reported where PPS is not generated when
start time is in past.

Example 1, "echo 0 0 0 2 0 > /sys/class/ptp/ptp0/period"

In the current implementation, a value of '0' is programmed
into Target time registers and PPS output is in pulse mode.
Eventually an interrupt which is triggered upon SYSTIM
register reaching Target time is not fired. Thus no PPS
output is generated.

Example 2, "echo 0 0 0 1 0 > /sys/class/ptp/ptp0/period"

Above case, a value of '0' is programmed into Target time
registers and PPS output is in clock mode. Here, HW tries to
catch-up the current time by incrementing Target Time
register. This catch-up time seem to vary according to
programmed PPS period time as per the HW design. In my
experiments, the delay ranged between few tens of seconds to
few minutes. The PPS output is only generated after the
Target time register reaches current time.

In my experiments, I also observed PPS stopped working with
below test and could not recover until module is removed and
loaded again.

1) echo 0 <future time> 0 1 0 > /sys/class/ptp/ptp1/period
2) echo 0 0 0 1 0 > /sys/class/ptp/ptp1/period
3) echo 0 0 0 1 0 > /sys/class/ptp/ptp1/period

After this PPS did not work even if i re-program with proper
values. I could only get this back working by reloading the
driver.

This patch takes care of calculating and programming
appropriate future time value into Target Time registers.

Fixes: 5e91c72e ("igc: Fix PPS delta between two synchronized end-points")
Signed-off-by: Aravindhan Gunasekaran <aravindhan.gunasekaran@intel.com>
Reviewed-by: Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>