igb: Fix missing time sync events

igb: Fix missing time sync events

[ Upstream commit ee14cc9ea19ba9678177e2224a9c58cce5937c73 ]

Fix "double" clearing of interrupts, which can cause external events
or timestamps to be missed.

The E1000_TSIRC Time Sync Interrupt Cause register can be cleared in two
ways, by either reading it or by writing '1' into the specific cause
bit. This is documented in section 8.16.1.

The following flow was used:
    1. read E1000_TSIRC into 'tsicr';
    2. handle the interrupts present into 'tsirc' and mark them in 'ack';
    3. write 'ack' into E1000_TSICR;

As both (1) and (3) will clear the interrupt cause, if the same
interrupt happens again between (1) and (3) it will be ignored,
causing events to be missed.

Remove the extra clear in (3).

Fixes: 00c65578b47b ("igb: enable internal PPS for the i210")
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>