wifi: cfg80211: sme: init n_channels before channels[] access

wifi: cfg80211: sme: init n_channels before channels[] access

With the __counted_by annocation in cfg80211_scan_request struct,
the "n_channels" struct member must be set before accessing the
"channels" array. Failing to do so will trigger a runtime warning
when enabling CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE.

Fixes: e3eac9f32ec0 ("wifi: cfg80211: Annotate struct cfg80211_scan_request with __counted_by")
Signed-off-by: Haoyu Li <lihaoyu499@gmail.com>
Link: https://patch.msgid.link/20241203152049.348806-1-lihaoyu499@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>