tree-wide: some more [static] related fixes

let's add [static] where it was missing so far

Drop [static] on parameters that can be NULL.

Add an assert() around parameters that have [static] and can't be NULL
hence.

Add some "const" where it was forgotten.

src/shared/ethtool-util.c

@@ -652,6 +652,8 @@ int ethtool_set_glinksettings(
         struct ifreq ifr = {};
         int r;
 
+        assert(advertise);
+
         if (autonegotiation != AUTONEG_DISABLE && memeqzero(advertise, sizeof(uint32_t) * N_ADVERTISE)) {
                 log_info("ethtool: autonegotiation is unset or enabled, the speed and duplex are not writable.");
                 return 0;