iwlwifi: fix antenna bitmask

The device only supports a maximum of three
antennas, and only three bits are used, the
fourth bit is the A-MPDU indicator.

The only consequence of this is reporting
invalid information in radiotap, so this
isn't an important change.

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