greybus: loopback: use dev_name to populate sysfsname

dev_name() will give a nice string representing the end0:X:Y:Z:W name
mitigating the need to pick apart the various nested data structures and
print out their various identifiers.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>