[media] fsl-viu: fix compiler warning

drivers/media/platform/fsl-viu.c: In function 'vidioc_s_fbuf':
drivers/media/platform/fsl-viu.c:867:32: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
This is fall-out from this commit:
commit e6eb28c2
Author: Hans Verkuil <hans.verkuil@cisco.com>
    [media] v4l2: make vidioc_s_fbuf const

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>