hwrng: mxc-rnga - Use the preferred format for kzalloc

According to Documentation/CodingStyle:

"The preferred form for passing a size of a struct is the following:

	p = kmalloc(sizeof(*p), ...);"

,so do as suggested.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>