[iwm] Remove unused field from iwm_rx_data. Use uint32_t instead of enum type.

The wantresp field in struct iwm_rx_data has never been used for anything,
so we can just delete it.

Apparently struct iwm_sf_cfg_cmd was compiled correctly (using a 32bit
value to represent the enum), but it still seems like a very bad idea to use
an enum type in a __packed struct.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7930