wifi: iwlwifi: mld: fix PM_SLEEP -Wundef warning

Config symbols are not defined if turned off, so need to
use #ifdef, not #if.

Fixes: d1e879ec ("wifi: iwlwifi: add iwlmld sub-driver")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Yedidya Ben Shimol <yedidya.ben.shimol@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20250401064530.612020bcdaad.I4e885e6646576e29fb236250a1b5038d3f14b08e@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>