iwlwifi: do not schedule tasklet when rcv unused irq

The nic controller's scheduler interrupt (CSR_INT_BIT_SCD) indicates
to the driver that scheduler finished to transmit the frame/frames.
This bit is not used and the tasklet should thus not be scheduled upon
its receipt.

Signed-off-by: Joonwoo Park <joonwpark81@gmail.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>