greybus: loopback: remove module specific identifier from debugfs name

The datapoint we are using to return metrics across modules and cports
shouldn't have a module identifier in it i.e.

/sys/kernel/debug/gb_loopback/raw_latency_endo0
not
/sys/kernel/debug/gb_loopback/raw_latency_endo0:X

This patch removes the module_id used up to this point. Including module_id
actually ends up making life harder in user-space so dropping it.

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