iwlwifi: move IWL_CMD_FAILED_MSK

This is common, not uCode API specific, so move
it to the transport together with the command
header struct definition.

Reviewed-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Reviewed-by: Guy Cohen <guy.cohen@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>