hwrng: core - use per-rng quality value instead of global setting

The current_quality variable exposed as a module parameter is
fundamentally broken: If it is set at boot time, it is overwritten once
the first hw rng device is loaded; if it is set at runtime, it is
without effect if the hw rng device had its quality value set to 0 (and
no default_quality was set); and if a new rng is selected, it gets
overwritten. Therefore, mark it as obsolete, and replace it by the
per-rng quality setting.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>