ieee80211: fix vht cap definitions

VHT_CAP_BEAMFORMER_ANTENNAS cap is actually defined in the draft as
VHT_CAP_BEAMFORMEE_STS_MAX, and its size is 3 bits long.

VHT_CAP_SOUNDING_DIMENSIONS is also 3 bits long.

Fix the definitions and change the cap masking accordingly.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>