hp_accel: do not call ACPI from invalid context

The LED on HP notebooks is connected through ACPI.  That unfortunately
means that it needs to be delayed by using schedule_work() to avoid
calling the ACPI interpreter from an invalid context.

[akpm@linux-foundation.org: use flush_work() rather than sort-of reimplementing it]
Signed-off-by: Pavel Machek <pavel@suse.cz>
Cc: Éric Piel <eric.piel@tremplin-utc.net>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>