Fix iwx(4) Tx ring array size which was one entry too short.

Fix iwx(4) Tx ring array size which was one entry too short.

Fortunately, this bug was harmless. The last Tx agg queue is never used
because ieee80211_classify() only returns TID values in the range 0 - 3.
And iterations over the txq array use nitems() to find the upper bound.

The possiblity of shrinking the txq array by 4 elements to get rid of
unused Tx agg queues could be investigated later.
For now, just fix the off-by-one error.

ok kettenis@