hwrng: imx-rngc - use dev_err_probe

Simplify the code by calling dev_err_probe instead of dev_err and return.

While at it, use the same device for all error messages.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Rouven Czerwinski <r.czerwinskis@pengutronix.de>
Reviewed-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>