tee: amdtee: remove redundant NULL check for pool

Remove NULL check for pool variable, since in the current
code path it is guaranteed to be non-NULL.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rijo Thomas <Rijo-john.Thomas@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>