greybus: loopback: drop redundant endo0 string from debugfs entry name

dev_name() will return the endo0 component of the string on it's own,
there's no need to include it in the snprintf() when construting the
debugfs name. This fixes 'endo0' appearing more than once in the debugfs
name - shamefully slipped through testing cb570c93783f
('greybus/loopback: use dev_name to populate sysfsname').

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>