e1000e: perform 10/100 adaptive IFS only on parts that support it

Adaptive IFS which involves writing to the Adaptive IFS Throttle register
was being done for all devices supported by the driver even though it is
not supported (i.e. the register doesn't even exist) on some devices. The
feature is supported on 8257x/82583 and ICH/PCH based devices, but not
on ESB2.

Signed-off-by: Bruce Allan <[e-mail]>
Signed-off-by: Jeff Kirsher <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/e1000e/82571.c

···
 	/* Set if manageability features are enabled. */
 	mac->arc_subsystem_valid = (er32(FWSM) & E1000_FWSM_MODE_MASK)
 	                ? true : false;
+	/* Adaptive IFS supported */
+	mac->adaptive_ifs = true;
 
 	/* check for link */
 	switch (hw->phy.media_type) {

drivers/net/e1000e/es2lan.c

···
 	/* Set if manageability features are enabled. */
 	mac->arc_subsystem_valid = (er32(FWSM) & E1000_FWSM_MODE_MASK)
                         ? true : false;
+	/* Adaptive IFS not supported */
+	mac->adaptive_ifs = false;
 
 	/* check for link */
 	switch (hw->phy.media_type) {

drivers/net/e1000e/hw.h

···
 
 	u8  forced_speed_duplex;
 
+	bool adaptive_ifs;
 	bool arc_subsystem_valid;
 	bool autoneg;
 	bool autoneg_failed;

drivers/net/e1000e/ich8lan.c

···
 		mac->rar_entry_count--;
 	/* Set if manageability features are enabled. */
 	mac->arc_subsystem_valid = true;
+	/* Adaptive IFS supported */
+	mac->adaptive_ifs = true;
 
 	/* LED operations */
 	switch (mac->type) {

drivers/net/e1000e/lib.c

···
 {
 	struct e1000_mac_info *mac = &hw->mac;
 
+	if (!mac->adaptive_ifs) {
+		e_dbg("Not in Adaptive IFS mode!\n");
+		goto out;
+	}
+
 	mac->current_ifs_val = 0;
 	mac->ifs_min_val = IFS_MIN;
 	mac->ifs_max_val = IFS_MAX;
···
 
 	mac->in_ifs_mode = false;
 	ew32(AIT, 0);
+out:
+	return;
 }
 
 /**
···
 void e1000e_update_adaptive(struct e1000_hw *hw)
 {
 	struct e1000_mac_info *mac = &hw->mac;
+
+	if (!mac->adaptive_ifs) {
+		e_dbg("Not in Adaptive IFS mode!\n");
+		goto out;
+	}
 
 	if ((mac->collision_delta * mac->ifs_ratio) > mac->tx_packet_delta) {
 		if (mac->tx_packet_delta > MIN_NUM_XMITS) {
···
 			ew32(AIT, 0);
 		}
 	}
+out:
+	return;
 }
 
 /**