omapfb/dss: wait_for_completion_interruptible_timeout expects long

wait_for_completion_timeout_interruptible returns long not unsigned long.
an appropriately typed variable is introduced and assignments fixed up.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>