[media] az6007: Comment the gate_ctl mutex

The mutex is there to protect the I2C gate. However, for some reason,
it is being called twice:

[ 2103.542796] usbcore: registered new interface driver dvb_usb_az6007
[ 2103.772392] az6007: drxk_gate_ctrl: enable
[ 2103.793900] az6007: drxk_gate_ctrl: enable

For now, let's just comment, to allow the driver to run.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>