[media] vp702x: fix locking of usb operations

Otherwise it is not obvious that vp702x_usb_in_op or vp702x_usb_out_op
will not interfere with any vp702x_usb_inout_op.

Note: This change is tested to compile only, as I don't have the
hardware.

Signed-off-by: Florian Mickler <florian@mickler.org>
Cc: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>