net: ti: icssg-prueth: Add missing icss_iep_put to error path

Analogously to prueth_remove, just also taking care for NULL'ing the
iep pointers.

Fixes: 186734c1 ("net: ti: icssg-prueth: add packet timestamping and ptp support")
Fixes: 443a2367 ("net: ti: icssg-prueth: am65x SR2.0 add 10M full duplex support")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: MD Danish Anwar <danishanwar@ti.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>