ieee80211: Check for copyout() errors in the SIOCG80211STATS handler

In preparation for annotating copyin() and related functions with
__result_use_check.

Reviewed by: bz, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43095

(cherry picked from commit d760d74dbed81638979e662130e24111b3a5db0d)