greybus: loopback_test: Cancel only the tests running on selected devices

When starting a loopback test, it cancels all currently running tests on
all loopback devices. When -m argument is given, which runs the test on
specific loopback devices, only the tests running on the selected
devices need to be cancelled before starting new tests.

Signed-off-by: Ryan Lim <limryan@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>