Get rid of convoluted macros in iwm's receive code path.

Get rid of convoluted macros in iwm's receive code path.

Since we now sync the entire Rx DMA buffer the SYNC_RESP_STRUCT() macro which
was used to sync just parts of the buffer can be replaced by assignments.
The SYNC_RESP_PTR() macro was already unused so just remove it.
The ADVANCE_RXQ() macro was used just once so expand it in place.