check_ethernet: fix up nits in toggle_usb_interface() method

Recently added "toggle USB interface" method wasn't consistently
using that same pattern to find/test ethernet devices.

Add quotes around use of the complete path for "authorized" field.

These changes were supposed to land with patchset 9 but the CQ picked up
patchset 8 instead. :/
   https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/608612/

BUG=chromium:677572
TEST=on local machine from console:
    touch /mnt/stateful_partition/.labmachine
    # disconnect ethernet cable
    bash -x /usr/local/bin/hooks/check_ethernet.hook
    # review output for all methods
    rm /mnt/stateful_partition/.labmachine

Change-Id: I4837c7e55676cb90b7a6d51dcefca1f51e5a243b
Reviewed-on: https://chromium-review.googlesource.com/674327
Commit-Ready: Grant Grundler <grundler@chromium.org>
Tested-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@google.com>
Reviewed-by: Grant Grundler <grundler@chromium.org>
1 file changed