iwlwifi: mvm: clean up SSN incrementation

Sometimes, due to SCD bug, we need to start the queue with an
higher SSN. The queue allocation function currently increments
the SSN in the packet itself, but it is pointless, since this
value is overridden later by iwl_mvm_tx_mpdu with the value
from mvmsta->tid_data[tid].seq_number. Updating tid data is
sufficient.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>