mac80211_hwsim: correct use of IEEE80211_VHT_CAP_RXSTBC_X

The mac80211_hwsim driver intends to say that it supports up to four
STBC receive streams, but instead it ends up saying something undefined.
The IEEE80211_VHT_CAP_RXSTBC_X macros aren't independent bits that can
be ORed together, but values.  In this case, _4 is the appropriate one
to use.

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