wifi: wireless: avoid strlen() in cfg80211_michael_mic_failure()

In 'cfg80211_michael_mic_failure()', avoid extra call to 'strlen()'
by using the value returned by 'sprintf()'. Compile tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Link: https://msgid.link/20240110054246.371651-1-dmantipov@yandex.ru


Signed-off-by: Johannes Berg <johannes.berg@intel.com>