modem: Use "$(...)" instead of `...` in set_madison_config

This deals properly with symlinks to filenames with spaces,
and adheres to our style guidelines.

BUG=chromium-os:22117
TEST=Adhoc; works if /var/lib/cromo symlinked to /tmp/foo\ bar

Change-Id: I1fff7d8b448cf940f90097eb7397a28b177fb53d
Reviewed-on: https://gerrit.chromium.org/gerrit/10990
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Tested-by: ttuttle <ttuttle@chromium.org>
diff --git a/modem b/modem
index c9a035d..c6a025a 100755
--- a/modem
+++ b/modem
@@ -239,7 +239,7 @@
 		return
 	fi
 
-	if [ `readlink -f $MADISON_CONFIG_GROUP_LOCATION` != \
+	if [ "$(readlink -f $MADISON_CONFIG_GROUP_LOCATION)" != \
 	     "$MADISON_CONFIG_GROUP_LOCATION" ] ; then
 		echo "$MADISON_CONFIG_GROUP_LOCATION points somewhere strange."
 		echo "Please reboot (or restart cromo) and try again."