V4L/DVB (9647): em28xx: void having two concurrent control URB's

Now that we have a polling task for IR, there's a race condition, since
IR can be polling while other operations are being doing. Also, we are
now sharing the same urb_buf for both read and write control urb
operations. So, we need a mutex.

Thanks to Davin Heitmueller <devin.heitmueller@gmail.com> for warning me.

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