iwlwifi: update iwl-commands.h for 3 stream support

This patch updates comments and constants to support 1, 2, or 3 spatial streams
in rate_n_flags .

Signed-off-by: Daniel Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>