drm: omapdrm: remove unused variable

Commit f9b9faf6d94dd29eab8c128905c7d091f955481d "drm: flip-work: change
drm_flip_work_init prototype" changed the drm_flip_work_init prototype
to a void function, which makes 'ret' an unused variable.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>