acpibtn_notify: ignore duplicate ACPI lid transitions

acpibtn_notify: ignore duplicate ACPI lid transitions
A Dell Precision 5510 produces two _LID events when the lid is closed.
This results in acpibtn_notify() adding two sleep tasks.  The laptop
suspends and resumes successfully, but on resume the second sleep
task runs and it goes right back to sleep.  Making the lid transition
a no-op when the existing value matches new _LID value works around
the problem.  OK miod@ jcs@