etc/usr/rc: fix argument passing for net drivers

The expected argument name would include the instance number, which
is not only redundant in many cases (FOOETHn_n=arg.., "n" being the
instance number) and conflicted with what netconf(8) does, but some
drivers need to be able to see the arguments for all instances of its
driver type--for example, dp8390 needs to know how many earlier
instances have been configured to use PCI.

Change-Id: I4830b823352722f554a032979464aba8b08fc166

etc/usr/rc

@@ -184,7 +184,7 @@ start|autoboot)
     for label in $(get_eth_labels); do
         driver=$(echo $label | sed 's/\(.*\)_.*/\1/')
         instance=$(echo $label | sed 's/.*_//')
-        eval arg=\$${label}_arg
+        eval arg=\$${driver}_arg
         if [ ! -z "$arg" ]; then arg=" $arg"; fi
         arg="-args \"instance=$instance$arg\""
         eval up $driver -label $label $arg -period 5HZ