When the diver is removed and all the resources are deallocated,
we should be iterating through the created devices only.

Currently, the iteration ends when vivid_devs[i] is NULL. Since
the array contains VIVID_MAX_DEVS elements, it will oops if
n_devs=VIVID_MAX_DEVS because in that case, no element is NULL.

Fixes: c88a96b0 ('[media] vivid: add core driver code')
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>