There's no need to keep both normal and BT statistics
versions around all the time in memory when we only
use a subset of both. So keep only the subsets that
we need in memory, depending on the debug config).

Also, in doing so, we can remove all the calls to
iwl_bt_statistics() in the driver as we'll just
access the copied statistics now.

Finally, also remove this call from the one place
where it might still be needed and automatically
detect what kind of statistics the device is sending
based on their size. This way, we don't need to keep
track of which devices do what any more, which is
good since this is subject to change based on the
ucode version (as some ucode even for non-BT devices
will in fact use BT statistics).

Warn upon encountering a statistics command from the
ucode that isn't known, so we will find such issues
earlier in the future.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Tested-by: Don Fry <donald.h.fry@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>