ixgbe: add write flush in ixgbe_clock_out_i2c_byte()

I2C access is timing critical. Always do a write flush after writing
to the I2CCTL register.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_phy.c

@@ -1457,6 +1457,7 @@ static s32 ixgbe_clock_out_i2c_byte(struct ixgbe_hw *hw, u8 data)
 	i2cctl = IXGBE_READ_REG(hw, IXGBE_I2CCTL);
 	i2cctl |= IXGBE_I2C_DATA_OUT;
 	IXGBE_WRITE_REG(hw, IXGBE_I2CCTL, i2cctl);
+	IXGBE_WRITE_FLUSH(hw);
 
 	return status;
 }