drm/vc4: bo: Fix unused variable warning

Commit 07a2975c65f2 ("drm/vc4: bo: Fix drmm_mutex_init memory hog")
removed the only use of the ret variable, but didn't remove the
variable itself leading to a unused variable warning.

Remove that variable.

Reported-by: Stephen Rothwell <[e-mail]>
Fixes: 07a2975c65f2 ("drm/vc4: bo: Fix drmm_mutex_init memory hog")
Reviewed-by: Maíra Canal <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vc4/vc4_bo.c

···
 {
 	struct vc4_dev *vc4 = to_vc4_dev(dev);
 	struct vc4_bo *bo;
-	int ret;
 
 	if (WARN_ON_ONCE(vc4->is_vc5))
 		return ERR_PTR(-ENODEV);