[media] sony-btf-mpx: v4l2_tuner struct is now constant

A patchset applied earlier changed v4l2_tuner struct parameter at
vidioc_s_tuner to const. Do this change at sony_btf_mpx_s_tuner(),
in order to remove this warning:

	drivers/media/i2c/sony-btf-mpx.c:335:2: warning: initialization from incompatible pointer type [enabled by default]
	drivers/media/i2c/sony-btf-mpx.c:335:2: warning: (near initialization for 'sony_btf_mpx_tuner_ops.s_tuner') [enabled by default]

Cc: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>