wifi: mac80211: fix the size calculation of ieee80211_ie_len_eht_cap()

Here should return the size of ieee80211_eht_cap_elem_fixed, so fix it.

Fixes: 820acc81

 ("mac80211: Add EHT capabilities to association/probe request")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://lore.kernel.org/r/06c13635fc03bcff58a647b8e03e9f01a74294bd.1679935259.git.ryder.lee@mediatek.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>