nl80211: suppress smatch warnings

smatch warns that we once checked request->ssids in two functions
and then unconditionally used it later again.

This is actually fine, because the code has a relationship between
attrs[NL80211_ATTR_SCAN_SSIDS], n_ssids and request->ssids, but
smatch isn't smart enough to realize that.

Suppress the warnings by always checking just n_ssids - that way
smatch won't know that request->ssids could be NULL, and since it
is only NULL when n_ssids is 0 we still check everything correctly.

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