igb: fix PHC stopping on max freq

igb: fix PHC stopping on max freq

For 82576 MAC type, max_adj is reported as 1000000000 ppb. However, if
this value is passed to igb_ptp_adjfreq_82576, incvalue overflows out of
INCVALUE_82576_MASK, resulting in setting of zero TIMINCA.incvalue, stopping
the PHC (instead of going at twice the nominal speed).

Fix the advertised max_adj value to the largest value hardware can handle.
As there is no min_adj value available (-max_adj is used instead), this will
also prevent stopping the clock intentionally. It's probably not a big deal,
other igb MAC types don't support stopping the clock, either.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>