iwlagn: simplify TX flags assignments

The first assignment of TX_CMD_FLG_SEQ_CTL_MSK for
ack-expected mgmt frames is overwritten later in
the function, so it's useless. Also, probe response
frames, BACK request and others there are mutually
exclusive so can be moved into an else branch.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>