hwrng: xgene - Improve error reporting for problems during .remove()

Returning an error value in a platform driver's remove callback results in
a generic error message being emitted by the driver core, but otherwise it
doesn't make a difference. The device goes away anyhow.

As the driver already emits a better error message than the core, suppress
the generic error message by returning zero unconditionally.

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