greybus: loopback: add module level sys/debug fs data points

Code this far has used the first connection's sysfs entry to present
variables intended to control the entire test - across multiple
connections. This patch changes that so that the module level variables
only appear at the end0:x level in sysfs.

Example:

Total counts for errors over the entire set of connections will be here
/sys/bus/greybus/devices/endo0:x/error_dev

In contrast an error for each connection will be presented like this
/sys/bus/greybus/devices/endo0:x:y:z:w/error_con

x = <module-id>
y = <interface-id>
z = <bundle-id>
w = <cport-id>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>