Last-TRB TD Size clear masks only 4 of the 5 TDSZ field bits (15 instead of 31) — controller mispredicts remaining packets on large transfers

Summary

'clear TD SIZE to zero' masks ~TDSZ_SET(15) but the field is 5 bits (31): when npkt had bit 4 set the last TRB reports 16 packets remaining - missed service opportunities, babble/short-packet misbehavior, completion timeouts on real silicon (QEMU model ignores TD Size so cannot manifest in guest). Upstream fixed by FreeBSD 4e792e431a92 (EuroBSDcon, real hardware). One-constant fix.