[iwn] Perform BUS_DMASYNC_PREREAD when initializing RX buffer

BUS_DMASYNC_PREREAD is required when setting up RX buffer, otherwise
data provided by card can be overwritten by data evicted from cache

Also use proper tag when setting up RX descriptor

Reviewed by: adrian, avos, ivadasz
Differential Revision: https://reviews.freebsd.org/D8717