hwrng: remember rng chosen by user

When a user chooses a rng source via sysfs attribute
this rng should be sticky, even when other sources
with better quality to register. This patch introduces
a simple way to remember the user's choice. This is
reflected by a new sysfs attribute file 'rng_selected'
which shows if the current rng has been chosen by
userspace. The new attribute file shows '1' for user
selected rng and '0' otherwise.

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>