hwrng: imx-rngc - reasonable timeout for initial seed

Set a more reasonable timeout for calculating the initial seed.

The reference manuals says that "The initial seed takes approximately
2,000,000 clock cycles." The rngc peripheral clock runs at >= 33.25MHz,
so seeding takes at most 60ms.

A timeout of 200ms is more appropriate than the current value of 3
seconds.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>