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 <[email protected]>
Tested-by: ttuttle <[email protected]>
1 file changed