drm/imx: imx-tve: Remove unused variable

'ret' is never used in tve_enable/tve_disable(), so just
remove it.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483544696-32707-1-git-send-email-fabio.estevam@nxp.com