check_ethernet: switch to /bin/sh (dash)

There are technical reasons why /bin/sh (currently dash) is the preferred
shell for ChromeOS, not bash.  Since checkbashism is reporting no errors
or warnings, should be simple to change right?

I expect so but wouldn't not be offended if sheriff or
trooper has to revert this change.

BUG=none
TEST=sudo apt-get install devscripts # outside of chroot
    checkbashism check_ethernet.hook # returns no output
    scp recover_duts/hooks/check_ethernet.hook  kefka.lab:/usr/local/bin/hooks/check_ethernet.hook
    ssh kefka.lab
    touch /mnt/stateful_partition/.labmachine
    # unplug link, wait for machine to reboot after ~2 minutes
    # log in again to review /var/log/messages and verify no
    #     shell errors are reported by check_ethernet.hook.
    #     (just the usually "attempting method ..." etc)
    rm /mnt/stateful_partition/.labmachine

Change-Id: I1ace53b339671caf3ba3a6359b5626b429459259
Reviewed-on: https://chromium-review.googlesource.com/1058443
Commit-Ready: Kirtika Ruchandani <kirtika@chromium.org>
Tested-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Kirtika Ruchandani <kirtika@chromium.org>
Reviewed-by: Grant Grundler <grundler@chromium.org>
1 file changed