mac80211_hwsim: Fix memory leak in hwsim_new_radio_nl()

hwsim_new_radio_nl() now copies the name attribute in order to add a
null-terminator.  mac80211_hwsim_new_radio() (indirectly) copies it
again into the net_device structure, so the first copy is not used or
freed later.  Free the first copy before returning.

Fixes: ff4dd73d

 ("mac80211_hwsim: check HWSIM_ATTR_RADIO_NAME length")
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>