hwrng: OMAP: store per-device data in per-device variables, not file statics

Encapsulate all of the RNG per-device state into a single per-device
structure record, as opposed to a set of per-driver file variables.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>