iwlwifi: mld: fix building with CONFIG_PM_SLEEP disabled

iwlwifi: mld: fix building with CONFIG_PM_SLEEP disabled

The newly added driver causes multiple build problems when CONFIG_PM_SLEEP is
disabled:

drivers/net/wireless/intel/iwlwifi/mld/mac80211.c:1982:12: error: 'iwl_mld_resume' defined but not used [-Werror=unused-function]
 1982 | static int iwl_mld_resume(struct ieee80211_hw *hw)
      |            ^~~~~~~~~~~~~~
drivers/net/wireless/intel/iwlwifi/mld/mac80211.c:1960:1: error: 'iwl_mld_suspend' defined but not used [-Werror=unused-function]
 1960 | iwl_mld_suspend(struct ieee80211_hw *hw, struct cfg80211_wowlan *wowlan)
      | ^~~~~~~~~~~~~~~
drivers/net/wireless/intel/iwlwifi/mld/mac80211.c:1946:13: error: 'iwl_mld_set_wakeup' defined but not used [-Werror=unused-function]
 1946 | static void iwl_mld_set_wakeup(struct ieee80211_hw *hw, bool enabled)
      |             ^~~~~~~~~~~~~~~~~~
drivers/net/wireless/intel/iwlwifi/mld/mac80211.c: In function 'iwl_mld_mac80211_start':
drivers/net/wireless/intel/iwlwifi/mld/mac80211.c:504:20: error: 'ret' is used uninitialized [-Werror=uninitialized]
  504 |         if (!in_d3 || ret) {
      |                    ^~
drivers/net/wireless/intel/iwlwifi/mld/mac80211.c:478:13: note: 'ret' was declared here
  478 |         int ret;
      |             ^~~

Hide the unused functions and make sure the 'ret' variable is not used
before the initialization.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20250325084340.378724-1-arnd@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>