module: don't create duplicate module ids

If we somehow get a hotplug event for a module id that we already have
created[1], don't try to create it again, or sysfs will complain loudly.
Instead, abort the creation properly.

[1] If, for example, you happened to run a script on a greybus emulator
twice in a row...

Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>
Reviewed-by: Alex Elder <elder@linaro.org>