[media] mxl111sf: remove an unused variable

We don't use this any more after 3be5bb71 "[media] mxl111sf: fix error
on stream stop in mxl111sf_ep6_streaming_ctrl()" and it makes GCC
complain.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>