wifi: iwlwifi: move dBm averaging function into utils

The function really is just a simple math helper. Move it into
iwl-utils.c so that it can also be used by iwlmld.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20250609211928.8cc965af6990.I09bb2137863e888efe756c92d8eb0271ec95456c@changeid