wifi: wext: avoid extra calls to strlen() in ieee80211_bss()

Since 'sprintf()' returns the number of characters emitted, an
extra calls to 'strlen()' in 'ieee80211_bss()' may be dropped.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Link: https://lore.kernel.org/r/20230912035522.15947-1-dmantipov@yandex.ru
Signed-off-by: Johannes Berg <johannes.berg@intel.com>