Rework firmware reference counting and error messages in wpi(4).

. Clarify the shared firmware abstraction in wpi_cached_firmware
  and its new sibling wpi_release_firmware.
. Fix typo in wpa_cache_firmware error branch leading to free NULL.
. Fix leak in wpi_load_firmware error branch.
. Sprinkle some kasserts to executably document invariants.
. A little KNF here and there.

Based on a patch from dh in PR kern/44144.

(riastradh)