iwlwifi: Move the core suspend function to iwl-agn-lib

The core suspend function is part of agn, iwl_mac80211 should only
handle mac80211 I/F operations.

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