mac80211: correct use of IEEE80211_VHT_CAP_RXSTBC_X

The mod mask for VHT capabilities intends to say that you can override
the number of STBC receive streams, and it does, but only by accident.
The IEEE80211_VHT_CAP_RXSTBC_X aren't bits to be set, but values (albeit
left-shifted).  ORing the bits together gets the right answer, but we
should use the _MASK macro here instead.

Signed-off-by: Danek Duvall <duvall@comfychair.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>