hwrng: mediatek - Setup default RNG quality

When hw_random device's quality is non-zero, it will automatically fill
the kernel's entropy pool at boot.  For the purpose, one conservative
quality value is being picked up as the default value.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>