mac80211: fix potential null pointer dereference

he_op is being dereferenced before it is null checked, hence there
is a potential null pointer dereference.

Fix this by moving the pointer dereference after he_op has been
properly null checked.

Notice that, currently, he_op is already being null checked before
calling this function at 4593:

4593	if (!he_oper ||
4594	    !ieee80211_verify_sta_he_mcs_support(sband, he_oper))
4595		ifmgd->flags |= IEEE80211_STA_DISABLE_HE;

but in case ieee80211_verify_sta_he_mcs_support is ever called
without verifying he_oper is not null, we will end up having a
null pointer dereference. So, we better don't take any chances.

Addresses-Coverity-ID: 1470068 ("Dereference before null check")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>