wifi: nl80211: Set num_sub_specs before looping through sub_specs

wifi: nl80211: Set num_sub_specs before looping through sub_specs

The processing of the struct cfg80211_sar_specs::sub_specs flexible
array requires its counter, num_sub_specs, to be assigned before the
loop in nl80211_set_sar_specs(). Leave the final assignment after the
loop in place in case fewer ended up in the array.

Fixes: aa4ec06c455d ("wifi: cfg80211: use __counted_by where appropriate")
Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/20250721183125.work.183-kees@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>