rt2x00: Fix IEEE80211_HT_CAP_RX_STBC assignment

IEEE80211_HT_CAP_RX_STBC is a 2 bit flag, and should thus
never be set as normal flag. Instead we must read the number
of RX paths from the EEPROM and set the IEEE80211_HT_CAP_RX_STBC
with the correct value (using the same logic as the number of TX
streams).

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>