iwlagn: fix the bit mask of a FH register in stop Tx DMA flow

When we stop the Tx DMA channels, we poll bits 16:31 in
FH_TSSR_RX_STATUS_REG. From 4965 and up, only the bits 16:26 are legal.
Bits 27:31 are not used and are always unset.
Polling them will lead to fail on timeout but since the timeout is quite
small, the stall was not felt.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>