Commit e1acdeb0 "ACPI / scan: Simplify acpi_match_device()"
introduced code that may lead to a NULL pointer dereference when
trying to unlock a mutex.  Fix that.

Fixes: e1acdeb0 "ACPI / scan: Simplify acpi_match_device()"
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>