imx-drm: imx-tve: remove unused variable

Commit f9b0e251 'drm: make mode_valid callback optional' left variable ret
unused; remove it.

This fixes the following compilation warning:

  drivers/staging/imx-drm/imx-tve.c: In function ‘imx_tve_connector_mode_valid’:
  drivers/staging/imx-drm/imx-tve.c:252:6: warning: unused variable ‘ret’ [-Wunused-variable]

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>