igb: refactor igb_ptp_adjfine_82580 to use diff_by_scaled_ppm

Driver's .adjfine interface functions use adjust_by_scaled_ppm and
diff_by_scaled_ppm introduced in commit 1060707e
("ptp: introduce helpers to adjust by scaled parts per million")
to calculate the required adjustment in a concise manner,
but not igb_ptp_adjfine_82580.
Fix it by introducing IGB_82580_BASE_PERIOD and changing function logic
to use diff_by_scaled_ppm.

Signed-off-by: Andrii Staikov <andrii.staikov@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@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>