iwlwifi: do not set the sequence control bit is not needed

Check the IEEE80211_TX_CTL_ASSIGN_SEQ flag from mac80211, then decide how to
set the TX_CMD_FLG_SEQ_CTL_MSK bit. Setting the wrong bit in BAR frame whill
make the firmware to increment the sequence number which is incorrect and
cause unknown behavior.

CC: stable@vger.kernel.org #3.0+
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>