Fix external radio_work deinit path

Fix external radio_work deinit path

The radio_work type was stored within the dynamically allocated
wpa_radio_work buffer and that buffer ended up getting freed before the
final use of the type string within radio_work_done(). This resulted in
freed memory being used for a debug print. Avoid this by freeing the
wpa_external_work instance after having completed radio_work_done() for
the related work.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>