hwrng: mxc-rnga - fix data_present API

Commit 45001e92, which added support for RNGA, ignored the previous commit
984e976f, which changed the data_present API.

Cc: Matt Mackall <mpm@selenic.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Alan Carvalho de Assis <acassis@gmail.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>