[iwm] Get rid of SYNC_RESP_STRUCT and SYNC_RESP_PTR macros.

• SYNC_RESP_STRUCT and SYNC_RESP_PTR originate from the OpenBSD version of iwm, and they weren't serving any real purpose in the FreeBSD port.

• We just do a single bus_dmamap_sync for syncing the complete received frame, instead of explicitly bus_dmamap_sync-ing subranges of the frame like in the OpenBSD iwm code.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7939