greybus: loopback: move sysfs entries to /sys/bus/greybus/devices/endo0

Currently we have sysfs entries that are created when the first incoming
connection is created as sub-nodes of the module associated with that
connection e.g. /sys/bus/grebus/devices/endo0:X where X is the module
identifier associated with the new connection. This is conceptually
incorrect since the sysfs entries we create actually aren't bound to a
module. Depending on the order connections are brought up we can also have
a situation where /sys/bus/greybus/devices/endo0:X has high-level control
sysfs data-points but /sys/bus/greybus/devices/endo0:Y does not. Rather
than needlessly replicate data-points across each endo0:X, endo0:Y, endo0:Z
sysfs directories it is more sensible to locate the entries in
/sys/bus/greybus/devices/endo0.

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>