staging: android/ion: testing the wrong variable

We're testing "pdev" but we intended to test "heap_pdev".  This is a
static checker fix and it's unlikely that anyone is affected by this
bug.

Fixes: 13439479 ('staging: ion: Add files for parsing the devicetree')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>