mac80211: Disable MCS > 7 in minstrel_ht when STA uses static SMPS

Disable multi stream rates (MCS > 7) when a STA is in static SMPS mode
since it has only one active rx chain. Hence, it doesn't even make
sense to sample multi stream rates.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>