iwlagn: add comments to iwl_bus / iwl_trans

Rename the recursive inclusion protection in iwl-bus.h while we are at it.

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>