tee: amdtee: fix an IS_ERR() vs NULL bug

The __get_free_pages() function does not return error pointers it returns
NULL so fix this condition to avoid a NULL dereference.

Fixes: 757cc3e9 ("tee: add AMD-TEE driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Rijo Thomas <Rijo-john.Thomas@amd.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>