acpi_ec(4): Don't probe erroneously if success occurred

In r360131, acpi_ec probe was changed to not clobber an error status prior to
several error cases that did not explicitly set the error variable before
goto'ing the exit path. However, I did not notice that the error variable was
not set to success in the success path. That caused all successful probes to
fail, which is obviously undesirable.

PR: 245778
Reported by: Neel Chauhan <neel AT neelc.org>, Evilham <contact AT evilham.com>
Tested by: Evilham
X-MFC-With: r360131