igb: Add MII write support

To facilitate running PHY parametric tests, add support for the SIOCSMIIREG
ioctl. This allows a userspace application to write to the PHY registers
to enable the test modes.
Signed-off-by: Jackie Jone <jackie.jone@alliedtelesis.co.nz>
Acked-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>
Link: https://lore.kernel.org/r/20240618213330.982046-1-anthony.l.nguyen@intel.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

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

@@ -9139,6 +9139,10 @@ static int igb_mii_ioctl(struct net_device *netdev, struct ifreq *ifr, int cmd)
 			return -EIO;
 		break;
 	case SIOCSMIIREG:
+		if (igb_write_phy_reg(&adapter->hw, data->reg_num & 0x1F,
+				      data->val_in))
+			return -EIO;
+		break;
 	default:
 		return -EOPNOTSUPP;
 	}