mac80211: enable easier manipulation of VHT beamforming caps

Introduce shift and mask defines for beamformee STS cap and number
of sounding dimensions cap as these can take any 3 bit value.
While at it also cleanup an unrequired parenthesis.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>