staging: r8188eu: replace sizeof(struct rtw_ieee80211_ht_cap) with sizeof(struct ieee80211_ht_cap)

Values of this expressions are equal, but ieee80211_ht_cap is library type.

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>