iwlwifi: move iwl_free_skb and mark it static

iwl_free_skb is used only in iwl-agn.c, move
it there and mark it static.

Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>