hwrng: imx-rngc - (trivial) simplify error prints

Remove the device name, it is added by the dev_...() routines.

Drop the error code as well. It will be shown by the driver core when
the probe operation failed.

Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>