ACPI / debugger: Fix regression introduced by IS_ERR_VALUE() removal

The FIFO unlocking mechanism in acpi_dbg has been broken by the
following commit:

  Commit: 287980e4
  Subject: remove lots of IS_ERR_VALUE abuses

It converted !IS_ERR_VALUE(ret) into !ret which was not entirely
correct. Fix the regression by taking ret > 0 into account too as
appropriate.

Fixes: 287980e4 (remove lots of IS_ERR_VALUE abuses)
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
[ rjw: Simplifications, changelog & subject massage ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>