driver: rng: Fix SMCCC TRNG crash

Fix a SMCCC TRNG null pointer crash due to a failed smccc feature
binding.

Fixes: 53355bb8 ("drivers: rng: add smccc trng driver")
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Weizhao Ouyang <o451686892@gmail.com>