hp_accel: fix race in device removal

The work queue has to be flushed after the device has been made
inaccessible.  The patch closes a window during which a work queue might
remain active after the device is removed and would then lead to ACPI
calls with undefined behavior.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Acked-by: Eric Piel <eric.piel@tremplin-utc.net>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: Pavel Herrmann <morpheus.ibis@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>