iwlwifi: move uCode helper functions to iwl-helpers.h

This patch adds iwl_free_fw_desc ucode helper function.
It also moves ucode helper functions to iwl-helpers.h.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>