mac80211: fix possible NULL pointerderef in obss pd code

he_spr_ie_elem is dereferenced before the NULL check. fix this by moving
the assignment after the check.

fixes commit 697f6c507c74 ("mac80211: propagate HE operation info into
bss_conf")

This was reported by the static code checker.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20190813070712.25509-1-john@phrozen.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>