igb: serialize access to the time sync interrupt registers

The time sync related interrupt registers may be manipulated from
different contexts. This patch protects the registers from being
asynchronously changed by the reset function.

Also, the patch removes a misleading comment. The reset function
is disabling a bunch of functions, not enabling them.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>