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>

drivers/net/ethernet/intel/igb/igb_ptp.c

@@ -67,6 +67,7 @@
 #define INCVALUE_82576_MASK		GENMASK(E1000_TIMINCA_16NS_SHIFT - 1, 0)
 #define INCVALUE_82576			(16u << IGB_82576_TSYNC_SHIFT)
 #define IGB_NBITS_82580			40
+#define IGB_82580_BASE_PERIOD		0x800000000
 
 static void igb_ptp_tx_hwtstamp(struct igb_adapter *adapter);
 static void igb_ptp_sdp_init(struct igb_adapter *adapter);
@@ -209,17 +210,11 @@ static int igb_ptp_adjfine_82580(struct ptp_clock_info *ptp, long scaled_ppm)
 	struct igb_adapter *igb = container_of(ptp, struct igb_adapter,
 					       ptp_caps);
 	struct e1000_hw *hw = &igb->hw;
-	int neg_adj = 0;
+	bool neg_adj;
 	u64 rate;
 	u32 inca;
 
-	if (scaled_ppm < 0) {
-		neg_adj = 1;
-		scaled_ppm = -scaled_ppm;
-	}
-	rate = scaled_ppm;
-	rate <<= 13;
-	rate = div_u64(rate, 15625);
+	neg_adj = diff_by_scaled_ppm(IGB_82580_BASE_PERIOD, scaled_ppm, &rate);
 
 	inca = rate & INCVALUE_MASK;
 	if (neg_adj)