Convert old make variable modifiers :U and :L to bmake :tu and :tl.

Reviewed by:	sjg
Block event interrupts when we don't need it as soon as possible.
Typically this feature is used for isochronous transfers.
This reduces the amount of XHCI interrupting.

MFC after:	1 week
Use the correct constant for 8000 IRQ/s.

MFC after:	1 week
Correct TRB type for multi TRB transfers of non-NORMAL type, like isochronous.
Only the first TRB should be markes as special. Subsequent ones should be
marked as NORMAL. Optimise away TD first variable.

MFC after:	1 week