greybus: connection: return error-valued pointer on creation errors

Return an ERR_PTR on errors when creating connections.

This allows driver probe to fail with a more informative error message
as not all connection creation errors are due to memory exhaustion.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>