hwrng: arm-smccc-trng - fix NO_ENTROPY handling

The SMCCC_RET_TRNG_NO_ENTROPY switch arm is never used because the
NO_ENTROPY return value is negative and negative values are handled
above the switch by immediately returning.

Fix by handling errors using a default arm in the switch.

Fixes: 0888d04b ("hwrng: Add Arm SMCCC TRNG based driver")
Signed-off-by: James Cowgill <james.cowgill@blaize.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>