iwlagn: clean up of transport layer

Move a few declarations needed by the transport layer to iwl-shared.h
Move iwl_cmd_meta, iwl_tx_queue and friends to the internal transport
header file.
Move iwl_device_cmd iwl_host_cmd and friends to iwl-trans.h since
these structs are used in the API to the transport layer.
Move get_cmd_string to the upper layer with a declaration in
iwl-shared.h.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>