MFC r250286:
Some fixes to snd_envy24ht(4) driver:
 - Allow DMA addresses anywhere in the lower 4GB; Envy24HT has a 32-bit DMA
engine, not 28-bit like Envy24.
 - Mark interrupt handler as MPSAFE, seems to be correctly synchronized.

PR:		kern/152378
Submitted by:	Jason Harmening <jason.harmening@gmail.com>