drivers/staging/iio: call mutex_unlock in error handling code

Adjust the error handling code so that it benefits from the call to
mutex_unlock at the end of the function.

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

// <smpl>
@rcu exists@
position p1;
expression E;
@@

mutex_lock@p1(E);
...
mutex_unlock(E);

@exists@
position rcu.p1;
expression E;
@@

*mutex_lock@p1(E);
... when != mutex_unlock(E);
?*return ...;
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>