hwrng: xgene - Simplify using dev_err_probe()

dev_err_probe simplifies the idiom:

	if (ret != -EPROBE_DEFER)
		dev_err(...)
	return ret;

, emits the error code in a human readable way and even yields a useful
entry in /sys/kernel/debug/devices_deferred in the EPROBE_DEFER case.

So simplify and at the same time improve the driver by using
dev_err_probe().

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>