wifi: rtw89: adjust init_he_cap() to add EHT cap into iftype_data

EHT capabilities are also stored in struct ieee80211_sband_iftype_data, so
adjust allocation of iftype_data as common part named init_he_eht_cap(),
and then init_eht_cap() can be added later. Don't change logic at all
by this patch.
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240112062640.36922-2-pkshih@realtek.com