tee: optee: fix a print error on rng probing

If we fail to probe the optee-rng device, we print a wrong message
referring to the firmware tpm.

Fixes: 476a3d58 ("tee: optee: don't fail probe because of optee-rng")
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>