iwlwifi: remove some unnecessary NULL checks

These pointers are an offset into the "sta" struct.  They're assigned
like this:

	const struct ieee80211_sta_vht_cap *vht_cap = &sta->vht_cap;

They're not the first member of the struct (->supp_rates[] is first) so
they can't be NULL.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>