tee: tee_get_drvdata(): fix description of return value

This patch fixes the description of tee_get_drvdata()'s return value.
It actually returns the driver_data pointer supplied to
tee_device_alloc() since the TEE subsystem was added to the kernel.

Fixes: 967c9cca ("tee: generic TEE subsystem")
Cc: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>