tee: correct max value for id allocation

The privileged dev id range is [TEE_NUM_DEVICES / 2, TEE_NUM_DEVICES).
The non-privileged dev id range is [0, TEE_NUM_DEVICES / 2).

So when finding a slot for them, need to use different max value.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>