V4L/DVB: drivers/media/video/pvrusb2: Add missing mutex_unlock

Add a mutex_unlock missing on the error path.  In the other functions in
the same file the locks and unlocks of this mutex appear to be balanced,
so it would seem that the same should hold in this case.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression E1;
@@

* mutex_lock(E1,...);
  <+... when != E1
  if (...) {
    ... when != E1
*   return ...;
  }
  ...+>
* mutex_unlock(E1,...);
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>