drivers/iio/adc/at91_adc.c: adjust inconsistent IS_ERR and PTR_ERR

Change the call to PTR_ERR to access the value just tested by IS_ERR.

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

)

// <smpl>
@@
expression e,e1;
@@

(
if (IS_ERR(e)) { ... PTR_ERR(e) ... }
|
if (IS_ERR(e=e1)) { ... PTR_ERR(e) ... }
|
*if (IS_ERR(e))
 { ...
*  PTR_ERR(e1)
   ... }
)
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>