mac80211_hwsim: fix smatch/sparse complaints

The code is fine in both cases as-is, but we can
write it slightly differently to fix smatch/sparse
complaints:
 * compare the skb pointer (which we use as a cookie)
   by casting the skb to unsigned long rather than the
   cookie to a pointer (fixes "different address spaces")
 * when transmitting, data->channel must be assigned,
   don't check it (fixes "dereferenced before check")

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