nl80211: correctly validate MU-MIMO groups

Since groups 0 and 63 are invalid, we should check for those bits.
Note that the 802.11 spec specifies the *bit* order, but the CPU
doesn't care about bit order since it can't address bits, so it's
always treating BIT(0) as the lowest bit within a byte.

Reported-by: Jan Fuchs <jan.fuchs@lancom.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>