staging: android: ion: dummy: fix dereference of ERR_PTR

ion_device_create() can fail and if it fails then it returns the error
value in ERR_PTR.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Reviewed-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>