mtip32xx: fixes and cleanups

Various low hanging fruit, kicked of by seeing one of the few remaining
req->special users.

Compile tested only.

Follow-Ups:
- Re: mtip32xx: fixes and cleanups
  - From: Jens Axboe
- [PATCH 9/9] mtip32xxx: use for_each_sg
  - From: Christoph Hellwig
- [PATCH 8/9] mtip32xx: don't use req->special
  - From: Christoph Hellwig
- [PATCH 7/9] mtip32xx: remove mtip_get_int_command
  - From: Christoph Hellwig
- [PATCH 6/9] mtip32xx: remove mtip_init_cmd_header
  - From: Christoph Hellwig
- [PATCH 5/9] mtip32xx: add missing endianess annotations on struct smart_attr
  - From: Christoph Hellwig
- [PATCH 4/9] mtip32xx: remove __force_bit2int
  - From: Christoph Hellwig
- [PATCH 3/9] mtip32xx: return a blk_status_t from mtip_send_trim
  - From: Christoph Hellwig
- [PATCH 2/9] mtip32xx: merge mtip_submit_request into mtip_queue_rq
  - From: Christoph Hellwig
- [PATCH 1/9] mtip32xx: move the blk_rq_map_sg call to mtip_hw_submit_io
  - From: Christoph Hellwig